@@ -771,7 +771,7 @@ supplied, otherwise the system functions.
-int pcre2_config(uint32_t what, void *where, PCRE2_SIZE length); +int pcre2_config(uint32_t what, void *where);
The function pcre2_config() makes it possible for a PCRE2 client to @@ -782,15 +782,17 @@ documentation has more details about these optional features.
The first argument for pcre2_config() specifies which information is required. The second argument is a pointer to memory into which the information -is placed, with the final argument giving the length of this memory in bytes. -For calls that return numerical values, where should point to -appropriately aligned memory, with length set to at least the "sizeof" -the data type. +is placed. If NULL is passed, the function returns the amount of memory that is +needed for the requested information. For calls that return numerical values, +the value is in bytes; when requesting these values, where should point +to appropriately aligned memory. For calls that return strings, the required +length is given in code units, not counting the terminating zero.
-The returned value from pcre2_config() is zero on success, or the -negative error code PCRE2_ERROR_BADOPTION if the value in the first argument is -not recognized. The following information is available: +When requesting information, the returned value from pcre2_config() is +non-negative on success, or the negative error code PCRE2_ERROR_BADOPTION if +the value in the first argument is not recognized. The following information is +available:
PCRE2_CONFIG_BSR@@ -806,10 +808,13 @@ compiling is available; otherwise it is set to zero.
PCRE2_CONFIG_JITTARGET-FIXME: this needs sorting out once JIT is implemented. -If JIT support is available, the string contains the name of the architecture -for which the JIT compiler is configured, for example "x86 32bit (little endian -+ unaligned)". If JIT support is not available, FIXME. +The where argument should point to a buffer that is at least 48 code +units long. (The exact length needed can be found by calling +pcre2_config() with where set to NULL.) The buffer is filled with a +string that contains the name of the architecture for which the JIT compiler is +configured, for example "x86 32bit (little endian + unaligned)". If JIT support +is not available, PCRE2_ERROR_BADOPTION is returned, otherwise the length of +the string, in code units, is returned.
PCRE2_CONFIG_LINKSIZE@@ -873,9 +878,12 @@ heap instead of recursive function calls. PCRE2_CONFIG_UNICODE_VERSION The where argument should point to a buffer that is at least 24 code -units long. If PCRE2 has been compiled without Unicode support, this is filled -with the text "Unicode not supported". Otherwise, the Unicode version string -(for example, "7.0.0") is returnd. The string is zero-terminated. +units long. (The exact length needed can be found by calling +pcre2_config() with where set to NULL.) If PCRE2 has been compiled +without Unicode support, the buffer is filled with the text "Unicode not +supported". Otherwise, the Unicode version string (for example, "7.0.0") is +inserted. The string is zero-terminated. The function returns the length of the +string in code units.
PCRE2_CONFIG_UNICODE@@ -884,9 +892,12 @@ otherwise it is set to zero. Unicode support implies UTF support.
PCRE2_CONFIG_VERSION-The where argument should point to a buffer that is at least 12 code -units long. It is filled with the PCRE2 version string, zero-terminated. - +The where argument should point to a buffer that is at least 12 code +units long. (The exact length needed can be found by calling +pcre2_config() with where set to NULL.) The buffer is filled with +the PCRE2 version string, zero-terminated. The length of the string (in code +units) is returned. +
pcre2_code *pcre2_compile(PCRE2_SPTR pattern, PCRE2_SIZE length, @@ -1354,20 +1365,20 @@ can be processed in different locales. The pcre2_pattern_info() function returns information about a compiled pattern. The first argument is a pointer to the compiled pattern. The second argument specifies which piece of information is required, and the third -argument is a pointer to a variable to receive the data. The yield of the -function is zero for success, or one of the following negative numbers: +argument is a pointer to a variable to receive the data. If the third argument +is NULL, the first argument is ignored, and the function returns the size in +bytes of the variable that is required for the information requested. +Otherwise, The yield of the function is zero for success, or one of the +following negative numbers:
PCRE2_ERROR_NULL the argument code was NULL - the argument where was NULL PCRE2_ERROR_BADMAGIC the "magic number" was not found PCRE2_ERROR_BADOPTION the value of what was invalid PCRE2_ERROR_UNSET the requested field is not setThe "magic number" is placed at the start of each compiled pattern as an simple -check against passing an arbitrary memory pointer. -Here is -a typical call of pcre2_pattern_info(), to obtain the length of the compiled -pattern: +check against passing an arbitrary memory pointer. Here is a typical call of +pcre2_pattern_info(), to obtain the length of the compiled pattern:
int rc; size_t length; @@ -2570,7 +2581,7 @@ Cambridge CB2 3QH, England.
REVISION
-Last updated: 14 October 2014 +Last updated: 16 October 2014
+
Copyright © 1997-2014 University of Cambridge.
diff --git a/doc/html/pcre2pattern.html b/doc/html/pcre2pattern.html new file mode 100644 index 0000000..4c8168b --- /dev/null +++ b/doc/html/pcre2pattern.html @@ -0,0 +1,3202 @@ + +pcre2pattern specification + + +pcre2pattern man page
++Return to the PCRE2 index page. +
++This page is part of the PCRE2 HTML documentation. It was generated +automatically from the original man page. If there is any nonsense in it, +please consult the man page, in case the conversion went wrong. +
+
+The syntax and semantics of the regular expressions that are supported by PCRE2 +are described in detail below. There is a quick-reference syntax summary in the +pcre2syntax +page. PCRE2 tries to match Perl syntax and semantics as closely as it can. +PCRE2 also supports some alternative regular expression syntax (which does not +conflict with the Perl syntax) in order to provide some compatibility with +regular expressions in Python, .NET, and Oniguruma. +
++Perl's regular expressions are described in its own documentation, and regular +expressions in general are covered in a number of books, some of which have +copious examples. Jeffrey Friedl's "Mastering Regular Expressions", published +by O'Reilly, covers regular expressions in great detail. This description of +PCRE2's regular expressions is intended as reference material. +
++This document discusses the patterns that are supported by PCRE2 when its main +matching function, pcre2_match(), is used. PCRE2 also has an alternative +matching function, pcre2_dfa_match(), which matches using a different +algorithm that is not Perl-compatible. Some of the features discussed below are +not available when DFA matching is used. The advantages and disadvantages of +the alternative function, and how it differs from the normal function, are +discussed in the +pcre2matching +page. +
++A number of options that can be passed to pcre2_compile() can also be set +by special items at the start of a pattern. These are not Perl-compatible, but +are provided to make these options accessible to pattern writers who are not +able to change the program that processes the pattern. Any number of these +items may appear, but they must all be together right at the start of the +pattern string, and the letters must be in upper case. +
++In the 8-bit and 16-bit PCRE2 libraries, characters may be coded either as +single code units, or as multiple UTF-8 or UTF-16 code units. UTF-32 can be +specified for the 32-bit library, in which case it constrains the character +values to valid Unicode code points. To process UTF strings, PCRE2 must be +built to include Unicode support. When using UTF strings you must either call +the compiling function with the PCRE2_UTF option, or the pattern must start +with the special sequence (*UTF), which is equivalent to setting the relevant +option. How setting a UTF mode affects pattern matching is mentioned in several +places below. There is also a summary of features in the +pcre2unicode +page. +
++Some applications that allow their users to supply patterns may wish to +restrict them to non-UTF data for security reasons. If the PCRE2_NEVER_UTF +option is set at compile time, (*UTF) is not allowed, and its appearance causes +an error. +
++Another special sequence that may appear at the start of a pattern is (*UCP). +This has the same effect as setting the PCRE2_UCP option: it causes sequences +such as \d and \w to use Unicode properties to determine character types, +instead of recognizing only characters with codes less than 128 via a lookup +table. +
++If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting +the PCRE2_NO_AUTO_POSSESS option. This stops PCRE2 from making quantifiers +possessive when what follows cannot match the repeated item. For example, by +default a+b is treated as a++b. For more details, see the +pcre2api +documentation. +
++If a pattern starts with (*NO_START_OPT), it has the same effect as setting the +PCRE2_NO_START_OPTIMIZE option. This disables several optimizations for quickly +reaching "no match" results. For more details, see the +pcre2api +documentation. +
++PCRE2 supports five different conventions for indicating line breaks in +strings: a single CR (carriage return) character, a single LF (linefeed) +character, the two-character sequence CRLF, any of the three preceding, or any +Unicode newline sequence. The +pcre2api +page has +further discussion +about newlines, and shows how to set the newline convention when calling +pcre2_compile(). +
++It is also possible to specify a newline convention by starting a pattern +string with one of the following five sequences: +
+ (*CR) carriage return + (*LF) linefeed + (*CRLF) carriage return, followed by linefeed + (*ANYCRLF) any of the three above + (*ANY) all Unicode newline sequences ++These override the default and the options given to the compiling function. For +example, on a Unix system where LF is the default newline sequence, the pattern +
+ (*CR)a.b ++changes the convention to CR. That pattern matches "a\nb" because LF is no +longer a newline. If more than one of these settings is present, the last one +is used. + +
+The newline convention affects where the circumflex and dollar assertions are +true. It also affects the interpretation of the dot metacharacter when +PCRE2_DOTALL is not set, and the behaviour of \N. However, it does not affect +what the \R escape sequence matches. By default, this is any Unicode newline +sequence, for Perl compatibility. However, this can be changed; see the +description of \R in the section entitled +"Newline sequences" +below. A change of \R setting can be combined with a change of newline +convention. +
++The caller of pcre2_match() can set a limit on the number of times the +internal match() function is called and on the maximum depth of +recursive calls. These facilities are provided to catch runaway matches that +are provoked by patterns with huge matching trees (a typical example is a +pattern with nested unlimited repeats) and to avoid running out of system stack +by too much recursion. When one of these limits is reached, pcre2_match() +gives an error return. The limits can also be set by items at the start of the +pattern of the form +
+ (*LIMIT_MATCH=d) + (*LIMIT_RECURSION=d) ++where d is any number of decimal digits. However, the value of the setting must +be less than the value set (or defaulted) by the caller of pcre2_match() +for it to have any effect. In other words, the pattern writer can lower the +limits set by the programmer, but not raise them. If there is more than one +setting of one of these limits, the lower value is used. + +
+PCRE2 can be compiled to run in an environment that uses EBCDIC as its +character code rather than ASCII or Unicode (typically a mainframe system). In +the sections below, character code values are ASCII or Unicode; in an EBCDIC +environment these characters may have different code values, and there are no +code points greater than 255. +
++A regular expression is a pattern that is matched against a subject string from +left to right. Most characters stand for themselves in a pattern, and match the +corresponding characters in the subject. As a trivial example, the pattern +
+ The quick brown fox ++matches a portion of a subject string that is identical to itself. When +caseless matching is specified (the PCRE2_CASELESS option), letters are matched +independently of case. + +
+The power of regular expressions comes from the ability to include alternatives +and repetitions in the pattern. These are encoded in the pattern by the use of +metacharacters, which do not stand for themselves but instead are +interpreted in some special way. +
++There are two different sets of metacharacters: those that are recognized +anywhere in the pattern except within square brackets, and those that are +recognized within square brackets. Outside square brackets, the metacharacters +are as follows: +
+ \ general escape character with several uses
+ ^ assert start of string (or line, in multiline mode)
+ $ assert end of string (or line, in multiline mode)
+ . match any character except newline (by default)
+ [ start character class definition
+ | start of alternative branch
+ ( start subpattern
+ ) end subpattern
+ ? extends the meaning of (
+ also 0 or 1 quantifier
+ also quantifier minimizer
+ * 0 or more quantifier
+ + 1 or more quantifier
+ also "possessive quantifier"
+ { start min/max quantifier
+
+Part of a pattern that is in square brackets is called a "character class". In
+a character class the only metacharacters are:
++ \ general escape character + ^ negate the class, but only if the first character + - indicates character range + [ POSIX character class (only if followed by POSIX syntax) + ] terminates the character class ++The following sections describe the use of each of the metacharacters. + +
+The backslash character has several uses. Firstly, if it is followed by a +character that is not a number or a letter, it takes away any special meaning +that character may have. This use of backslash as an escape character applies +both inside and outside character classes. +
++For example, if you want to match a * character, you write \* in the pattern. +This escaping action applies whether or not the following character would +otherwise be interpreted as a metacharacter, so it is always safe to precede a +non-alphanumeric with backslash to specify that it stands for itself. In +particular, if you want to match a backslash, you write \\. +
++In a UTF mode, only ASCII numbers and letters have any special meaning after a +backslash. All other characters (in particular, those whose codepoints are +greater than 127) are treated as literals. +
++If a pattern is compiled with the PCRE2_EXTENDED option, most white space in +the pattern (other than in a character class), and characters between a # +outside a character class and the next newline, inclusive, are ignored. An +escaping backslash can be used to include a white space or # character as part +of the pattern. +
++If you want to remove the special meaning from a sequence of characters, you +can do so by putting them between \Q and \E. This is different from Perl in +that $ and @ are handled as literals in \Q...\E sequences in PCRE2, whereas +in Perl, $ and @ cause variable interpolation. Note the following examples: +
+ Pattern PCRE2 matches Perl matches + + \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz + \Qabc\$xyz\E abc\$xyz abc\$xyz + \Qabc\E\$\Qxyz\E abc$xyz abc$xyz ++The \Q...\E sequence is recognized both inside and outside character classes. +An isolated \E that is not preceded by \Q is ignored. If \Q is not followed +by \E later in the pattern, the literal interpretation continues to the end of +the pattern (that is, \E is assumed at the end). If the isolated \Q is inside +a character class, this causes an error, because the character class is not +terminated. + +
+A second use of backslash provides a way of encoding non-printing characters +in patterns in a visible manner. There is no restriction on the appearance of +non-printing characters in a pattern, but when a pattern is being prepared by +text editing, it is often easier to use one of the following escape sequences +than the binary character it represents: +
+ \a alarm, that is, the BEL character (hex 07)
+ \cx "control-x", where x is any ASCII character
+ \e escape (hex 1B)
+ \f form feed (hex 0C)
+ \n linefeed (hex 0A)
+ \r carriage return (hex 0D)
+ \t tab (hex 09)
+ \0dd character with octal code 0dd
+ \ddd character with octal code ddd, or back reference
+ \o{ddd..} character with octal code ddd..
+ \xhh character with hex code hh
+ \x{hhh..} character with hex code hhh.. (default mode)
+ \uhhhh character with hex code hhhh (when PCRE2_ALT_BSUX is set)
+
+The precise effect of \cx on ASCII characters is as follows: if x is a lower
+case letter, it is converted to upper case. Then bit 6 of the character (hex
+40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A (A is 41, Z is 5A),
+but \c{ becomes hex 3B ({ is 7B), and \c; becomes hex 7B (; is 3B). If the
+code unit following \c has a value greater than 127, a compile-time error
+occurs. This locks out non-ASCII characters in all modes.
+
++The \c facility was designed for use with ASCII characters, but with the +extension to Unicode it is even less useful than it once was. It is, however, +recognized when PCRE2 is compiled in EBCDIC mode, where data items are always +bytes. In this mode, all values are valid after \c. If the next character is a +lower case letter, it is converted to upper case. Then the 0xc0 bits of the +byte are inverted. Thus \cA becomes hex 01, as in ASCII (A is C1), but because +the EBCDIC letters are disjoint, \cZ becomes hex 29 (Z is E9), and other +characters also generate different values. +
++After \0 up to two further octal digits are read. If there are fewer than two +digits, just those that are present are used. Thus the sequence \0\x\07 +specifies two binary zeros followed by a BEL character (code value 7). Make +sure you supply two digits after the initial zero if the pattern character that +follows is itself an octal digit. +
++The escape \o must be followed by a sequence of octal digits, enclosed in +braces. An error occurs if this is not the case. This escape is a recent +addition to Perl; it provides way of specifying character code points as octal +numbers greater than 0777, and it also allows octal numbers and back references +to be unambiguously specified. +
++For greater clarity and unambiguity, it is best to avoid following \ by a +digit greater than zero. Instead, use \o{} or \x{} to specify character +numbers, and \g{} to specify back references. The following paragraphs +describe the old, ambiguous syntax. +
++The handling of a backslash followed by a digit other than 0 is complicated, +and Perl has changed in recent releases, causing PCRE2 also to change. Outside +a character class, PCRE2 reads the digit and any following digits as a decimal +number. If the number is less than 8, or if there have been at least that many +previous capturing left parentheses in the expression, the entire sequence is +taken as a back reference. A description of how this works is given +later, +following the discussion of +parenthesized subpatterns. +
++Inside a character class, or if the decimal number following \ is greater than +7 and there have not been that many capturing subpatterns, PCRE2 handles \8 +and \9 as the literal characters "8" and "9", and otherwise re-reads up to +three octal digits following the backslash, using them to generate a data +character. Any subsequent digits stand for themselves. For example: +
+ \040 is another way of writing an ASCII space + \40 is the same, provided there are fewer than 40 previous capturing subpatterns + \7 is always a back reference + \11 might be a back reference, or another way of writing a tab + \011 is always a tab + \0113 is a tab followed by the character "3" + \113 might be a back reference, otherwise the character with octal code 113 + \377 might be a back reference, otherwise the value 255 (decimal) + \81 is either a back reference, or the two characters "8" and "1" ++Note that octal values of 100 or greater that are specified using this syntax +must not be introduced by a leading zero, because no more than three octal +digits are ever read. + +
+By default, after \x that is not followed by {, from zero to two hexadecimal +digits are read (letters can be in upper or lower case). Any number of +hexadecimal digits may appear between \x{ and }. If a character other than +a hexadecimal digit appears between \x{ and }, or if there is no terminating +}, an error occurs. +
++If the PCRE2_ALT_BSUX option is set, the interpretation of \x is as just +described only when it is followed by two hexadecimal digits. Otherwise, it +matches a literal "x" character. In this mode mode, support for code points +greater than 256 is provided by \u, which must be followed by four hexadecimal +digits; otherwise it matches a literal "u" character. +
++Characters whose value is less than 256 can be defined by either of the two +syntaxes for \x (or by \u in PCRE2_ALT_BSUX mode). There is no difference in +the way they are handled. For example, \xdc is exactly the same as \x{dc} (or +\u00dc in PCRE2_ALT_BSUX mode). +
++Characters that are specified using octal or hexadecimal numbers are +limited to certain values, as follows: +
+ 8-bit non-UTF mode less than 0x100 + 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint + 16-bit non-UTF mode less than 0x10000 + 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint + 32-bit non-UTF mode less than 0x100000000 + 32-bit UTF-32 mode less than 0x10ffff and a valid codepoint ++Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called +"surrogate" codepoints), and 0xffef. + +
+All the sequences that define a single character value can be used both inside +and outside character classes. In addition, inside a character class, \b is +interpreted as the backspace character (hex 08). +
++\N is not allowed in a character class. \B, \R, and \X are not special +inside a character class. Like other unrecognized alphabetic escape sequences, +they cause an error. Outside a character class, these sequences have different +meanings. +
++In Perl, the sequences \l, \L, \u, and \U are recognized by its string +handler and used to modify the case of following characters. By default, PCRE2 +does not support these escape sequences. However, if the PCRE2_ALT_BSUX option +is set, \U matches a "U" character, and \u can be used to define a character +by code point, as described in the previous section. +
++The sequence \g followed by an unsigned or a negative number, optionally +enclosed in braces, is an absolute or relative back reference. A named back +reference can be coded as \g{name}. Back references are discussed +later, +following the discussion of +parenthesized subpatterns. +
++For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or +a number enclosed either in angle brackets or single quotes, is an alternative +syntax for referencing a subpattern as a "subroutine". Details are discussed +later. +Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not +synonymous. The former is a back reference; the latter is a +subroutine +call. +
++Another use of backslash is for specifying generic character types: +
+ \d any decimal digit + \D any character that is not a decimal digit + \h any horizontal white space character + \H any character that is not a horizontal white space character + \s any white space character + \S any character that is not a white space character + \v any vertical white space character + \V any character that is not a vertical white space character + \w any "word" character + \W any "non-word" character ++There is also the single sequence \N, which matches a non-newline character. +This is the same as +the "." metacharacter +when PCRE2_DOTALL is not set. Perl also uses \N to match characters by name; +PCRE2 does not support this. + +
+Each pair of lower and upper case escape sequences partitions the complete set +of characters into two disjoint sets. Any given character matches one, and only +one, of each pair. The sequences can appear both inside and outside character +classes. They each match one character of the appropriate type. If the current +matching point is at the end of the subject string, all of them fail, because +there is no character to match. +
++The default \s characters are HT (9), LF (10), VT (11), FF (12), CR (13), and +space (32), which are defined as white space in the "C" locale. This list may +vary if locale-specific matching is taking place. For example, in some locales +the "non-breaking space" character (\xA0) is recognized as white space, and in +others the VT character is not. +
++A "word" character is an underscore or any character that is a letter or digit. +By default, the definition of letters and digits is controlled by PCRE2's +low-valued character tables, and may vary if locale-specific matching is taking +place (see +"Locale support" +in the +pcre2api +page). For example, in a French locale such as "fr_FR" in Unix-like systems, +or "french" in Windows, some character codes greater than 127 are used for +accented letters, and these are then matched by \w. The use of locales with +Unicode is discouraged. +
++By default, characters whose code points are greater than 127 never match \d, +\s, or \w, and always match \D, \S, and \W, although this may vary for +characters in the range 128-255 when locale-specific matching is happening. +These escape sequences retain their original meanings from before Unicode +support was available, mainly for efficiency reasons. If PCRE2 is compiled with +Unicode support, and the PCRE2_UCP option is set, the behaviour is changed so +that Unicode properties are used to determine character types, as follows: +
+ \d any character that matches \p{Nd} (decimal digit)
+ \s any character that matches \p{Z} or \h or \v
+ \w any character that matches \p{L} or \p{N}, plus underscore
+
+The upper case escapes match the inverse sets of characters. Note that \d
+matches only decimal digits, whereas \w matches any Unicode digit, as well as
+any Unicode letter, and underscore. Note also that PCRE2_UCP affects \b, and
+\B because they are defined in terms of \w and \W. Matching these sequences
+is noticeably slower when PCRE2_UCP is set.
+
++The sequences \h, \H, \v, and \V, in contrast to the other sequences, which +match only ASCII characters by default, always match a specific list of code +points, whether or not PCRE2_UCP is set. The horizontal space characters are: +
+ U+0009 Horizontal tab (HT) + U+0020 Space + U+00A0 Non-break space + U+1680 Ogham space mark + U+180E Mongolian vowel separator + U+2000 En quad + U+2001 Em quad + U+2002 En space + U+2003 Em space + U+2004 Three-per-em space + U+2005 Four-per-em space + U+2006 Six-per-em space + U+2007 Figure space + U+2008 Punctuation space + U+2009 Thin space + U+200A Hair space + U+202F Narrow no-break space + U+205F Medium mathematical space + U+3000 Ideographic space ++The vertical space characters are: +
+ U+000A Linefeed (LF) + U+000B Vertical tab (VT) + U+000C Form feed (FF) + U+000D Carriage return (CR) + U+0085 Next line (NEL) + U+2028 Line separator + U+2029 Paragraph separator ++In 8-bit, non-UTF-8 mode, only the characters with code points less than 256 +are relevant. + +
+Outside a character class, by default, the escape sequence \R matches any +Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the +following: +
+ (?>\r\n|\n|\x0b|\f|\r|\x85) ++This is an example of an "atomic group", details of which are given +below. +This particular group matches either the two-character sequence CR followed by +LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab, +U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next +line, U+0085). The two-character sequence is treated as a single unit that +cannot be split. + +
+In other modes, two additional characters whose codepoints are greater than 255 +are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029). +Unicode support is not needed for these characters to be recognized. +
++It is possible to restrict \R to match only CR, LF, or CRLF (instead of the +complete set of Unicode line endings) by setting the option PCRE2_BSR_ANYCRLF +at compile time. (BSR is an abbrevation for "backslash R".) This can be made +the default when PCRE2 is built; if this is the case, the other behaviour can +be requested via the PCRE2_BSR_UNICODE option. It is also possible to specify +these settings by starting a pattern string with one of the following +sequences: +
+ (*BSR_ANYCRLF) CR, LF, or CRLF only + (*BSR_UNICODE) any Unicode newline sequence ++These override the default and the options given to the compiling function. +Note that these special settings, which are not Perl-compatible, are recognized +only at the very start of a pattern, and that they must be in upper case. If +more than one of them is present, the last one is used. They can be combined +with a change of newline convention; for example, a pattern can start with: +
+ (*ANY)(*BSR_ANYCRLF) ++They can also be combined with the (*UTF) or (*UCP) special sequences. Inside a +character class, \R is treated as an unrecognized escape sequence, and causes +an error. + +
+When PCRE2 is built with Unicode support, three additional escape sequences +that match characters with specific properties are available. In 8-bit +non-UTF-8 mode, these sequences are of course limited to testing characters +whose codepoints are less than 256, but they do work in this mode. The extra +escape sequences are: +
+ \p{xx} a character with the xx property
+ \P{xx} a character without the xx property
+ \X a Unicode extended grapheme cluster
+
+The property names represented by xx above are limited to the Unicode
+script names, the general category properties, "Any", which matches any
+character (including newline), and some special PCRE2 properties (described
+in the
+next section).
+Other Perl properties such as "InMusicalSymbols" are not supported by PCRE2.
+Note that \P{Any} does not match any characters, so always causes a match
+failure.
+
++Sets of Unicode characters are defined as belonging to certain scripts. A +character from one of these sets can be matched using a script name. For +example: +
+ \p{Greek}
+ \P{Han}
+
+Those that are not part of an identified script are lumped together as
+"Common". The current list of scripts is:
+
++Arabic, +Armenian, +Avestan, +Balinese, +Bamum, +Bassa_Vah, +Batak, +Bengali, +Bopomofo, +Brahmi, +Braille, +Buginese, +Buhid, +Canadian_Aboriginal, +Carian, +Caucasian_Albanian, +Chakma, +Cham, +Cherokee, +Common, +Coptic, +Cuneiform, +Cypriot, +Cyrillic, +Deseret, +Devanagari, +Duployan, +Egyptian_Hieroglyphs, +Elbasan, +Ethiopic, +Georgian, +Glagolitic, +Gothic, +Grantha, +Greek, +Gujarati, +Gurmukhi, +Han, +Hangul, +Hanunoo, +Hebrew, +Hiragana, +Imperial_Aramaic, +Inherited, +Inscriptional_Pahlavi, +Inscriptional_Parthian, +Javanese, +Kaithi, +Kannada, +Katakana, +Kayah_Li, +Kharoshthi, +Khmer, +Khojki, +Khudawadi, +Lao, +Latin, +Lepcha, +Limbu, +Linear_A, +Linear_B, +Lisu, +Lycian, +Lydian, +Mahajani, +Malayalam, +Mandaic, +Manichaean, +Meetei_Mayek, +Mende_Kikakui, +Meroitic_Cursive, +Meroitic_Hieroglyphs, +Miao, +Modi, +Mongolian, +Mro, +Myanmar, +Nabataean, +New_Tai_Lue, +Nko, +Ogham, +Ol_Chiki, +Old_Italic, +Old_North_Arabian, +Old_Permic, +Old_Persian, +Old_South_Arabian, +Old_Turkic, +Oriya, +Osmanya, +Pahawh_Hmong, +Palmyrene, +Pau_Cin_Hau, +Phags_Pa, +Phoenician, +Psalter_Pahlavi, +Rejang, +Runic, +Samaritan, +Saurashtra, +Sharada, +Shavian, +Siddham, +Sinhala, +Sora_Sompeng, +Sundanese, +Syloti_Nagri, +Syriac, +Tagalog, +Tagbanwa, +Tai_Le, +Tai_Tham, +Tai_Viet, +Takri, +Tamil, +Telugu, +Thaana, +Thai, +Tibetan, +Tifinagh, +Tirhuta, +Ugaritic, +Vai, +Warang_Citi, +Yi. +
++Each character has exactly one Unicode general category property, specified by +a two-letter abbreviation. For compatibility with Perl, negation can be +specified by including a circumflex between the opening brace and the property +name. For example, \p{^Lu} is the same as \P{Lu}. +
++If only one letter is specified with \p or \P, it includes all the general +category properties that start with that letter. In this case, in the absence +of negation, the curly brackets in the escape sequence are optional; these two +examples have the same effect: +
+ \p{L}
+ \pL
+
+The following general category property codes are supported:
++ C Other + Cc Control + Cf Format + Cn Unassigned + Co Private use + Cs Surrogate + + L Letter + Ll Lower case letter + Lm Modifier letter + Lo Other letter + Lt Title case letter + Lu Upper case letter + + M Mark + Mc Spacing mark + Me Enclosing mark + Mn Non-spacing mark + + N Number + Nd Decimal number + Nl Letter number + No Other number + + P Punctuation + Pc Connector punctuation + Pd Dash punctuation + Pe Close punctuation + Pf Final punctuation + Pi Initial punctuation + Po Other punctuation + Ps Open punctuation + + S Symbol + Sc Currency symbol + Sk Modifier symbol + Sm Mathematical symbol + So Other symbol + + Z Separator + Zl Line separator + Zp Paragraph separator + Zs Space separator ++The special property L& is also supported: it matches a character that has +the Lu, Ll, or Lt property, in other words, a letter that is not classified as +a modifier or "other". + +
+The Cs (Surrogate) property applies only to characters in the range U+D800 to +U+DFFF. Such characters are not valid in Unicode strings and so +cannot be tested by PCRE2, unless UTF validity checking has been turned off +(see the discussion of PCRE2_NO_UTF_CHECK in the +pcre2api +page). Perl does not support the Cs property. +
++The long synonyms for property names that Perl supports (such as \p{Letter}) +are not supported by PCRE2, nor is it permitted to prefix any of these +properties with "Is". +
++No character that is in the Unicode table has the Cn (unassigned) property. +Instead, this property is assumed for any code point that is not in the +Unicode table. +
++Specifying caseless matching does not affect these escape sequences. For +example, \p{Lu} always matches only upper case letters. This is different from +the behaviour of current versions of Perl. +
++Matching characters by Unicode property is not fast, because PCRE2 has to do a +multistage table lookup in order to find a character's property. That is why +the traditional escape sequences such as \d and \w do not use Unicode +properties in PCRE2 by default, though you can make them do so by setting the +PCRE2_UCP option or by starting the pattern with (*UCP). +
++The \X escape matches any number of Unicode characters that form an "extended +grapheme cluster", and treats the sequence as an atomic group +(see below). +Unicode supports various kinds of composite character by giving each character +a grapheme breaking property, and having rules that use these properties to +define the boundaries of extended grapheme clusters. \X always matches at +least one character. Then it decides whether to add additional characters +according to the following rules for ending a cluster: +
++1. End at the end of the subject string. +
++2. Do not end between CR and LF; otherwise end after any control character. +
++3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters +are of five types: L, V, T, LV, and LVT. An L character may be followed by an +L, V, LV, or LVT character; an LV or V character may be followed by a V or T +character; an LVT or T character may be follwed only by a T character. +
++4. Do not end before extending characters or spacing marks. Characters with +the "mark" property always have the "extend" grapheme breaking property. +
++5. Do not end after prepend characters. +
++6. Otherwise, end the cluster. +
++As well as the standard Unicode properties described above, PCRE2 supports four +more that make it possible to convert traditional escape sequences such as \w +and \s to use Unicode properties. PCRE2 uses these non-standard, non-Perl +properties internally when PCRE2_UCP is set. However, they may also be used +explicitly. These properties are: +
+ Xan Any alphanumeric character + Xps Any POSIX space character + Xsp Any Perl space character + Xwd Any Perl "word" character ++Xan matches characters that have either the L (letter) or the N (number) +property. Xps matches the characters tab, linefeed, vertical tab, form feed, or +carriage return, and any other character that has the Z (separator) property. +Xsp is the same as Xps; in PCRE1 it used to exclude vertical tab, for Perl +compatibility, but Perl changed. Xwd matches the same characters as Xan, plus +underscore. + +
+There is another non-standard property, Xuc, which matches any character that +can be represented by a Universal Character Name in C++ and other programming +languages. These are the characters $, @, ` (grave accent), and all characters +with Unicode code points greater than or equal to U+00A0, except for the +surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are +excluded. (Universal Character Names are of the form \uHHHH or \UHHHHHHHH +where H is a hexadecimal digit. Note that the Xuc property does not match these +sequences but the characters that they represent.) +
++The escape sequence \K causes any previously matched characters not to be +included in the final matched sequence. For example, the pattern: +
+ foo\Kbar ++matches "foobar", but reports that it has matched "bar". This feature is +similar to a lookbehind assertion +(described below). +However, in this case, the part of the subject before the real match does not +have to be of fixed length, as lookbehind assertions do. The use of \K does +not interfere with the setting of +captured substrings. +For example, when the pattern +
+ (foo)\Kbar ++matches "foobar", the first substring is still set to "foo". + +
+Perl documents that the use of \K within assertions is "not well defined". In +PCRE2, \K is acted upon when it occurs inside positive assertions, but is +ignored in negative assertions. Note that when a pattern such as (?=ab\K) +matches, the reported start of the match can be greater than the end of the +match. +
++The final use of backslash is for certain simple assertions. An assertion +specifies a condition that has to be met at a particular point in a match, +without consuming any characters from the subject string. The use of +subpatterns for more complicated assertions is described +below. +The backslashed assertions are: +
+ \b matches at a word boundary + \B matches when not at a word boundary + \A matches at the start of the subject + \Z matches at the end of the subject + also matches before a newline at the end of the subject + \z matches only at the end of the subject + \G matches at the first matching position in the subject ++Inside a character class, \b has a different meaning; it matches the backspace +character. If any other of these assertions appears in a character class, an +"invalid escape sequence" error is generated. + +
+A word boundary is a position in the subject string where the current character +and the previous character do not both match \w or \W (i.e. one matches +\w and the other matches \W), or the start or end of the string if the +first or last character matches \w, respectively. In a UTF mode, the meanings +of \w and \W can be changed by setting the PCRE2_UCP option. When this is +done, it also affects \b and \B. Neither PCRE2 nor Perl has a separate "start +of word" or "end of word" metasequence. However, whatever follows \b normally +determines which it is. For example, the fragment \ba matches "a" at the start +of a word. +
++The \A, \Z, and \z assertions differ from the traditional circumflex and +dollar (described in the next section) in that they only ever match at the very +start and end of the subject string, whatever options are set. Thus, they are +independent of multiline mode. These three assertions are not affected by the +PCRE2_NOTBOL or PCRE2_NOTEOL options, which affect only the behaviour of the +circumflex and dollar metacharacters. However, if the startoffset +argument of pcre2_match() is non-zero, indicating that matching is to +start at a point other than the beginning of the subject, \A can never match. +The difference between \Z and \z is that \Z matches before a newline at the +end of the string as well as at the very end, whereas \z matches only at the +end. +
++The \G assertion is true only when the current matching position is at the +start point of the match, as specified by the startoffset argument of +pcre2_match(). It differs from \A when the value of startoffset is +non-zero. By calling pcre2_match() multiple times with appropriate +arguments, you can mimic Perl's /g option, and it is in this kind of +implementation where \G can be useful. +
++Note, however, that PCRE2's interpretation of \G, as the start of the current +match, is subtly different from Perl's, which defines it as the end of the +previous match. In Perl, these can be different when the previously matched +string was empty. Because PCRE2 does just one match at a time, it cannot +reproduce this behaviour. +
++If all the alternatives of a pattern begin with \G, the expression is anchored +to the starting match position, and the "anchored" flag is set in the compiled +regular expression. +
++The circumflex and dollar metacharacters are zero-width assertions. That is, +they test for a particular condition being true without consuming any +characters from the subject string. +
++Outside a character class, in the default matching mode, the circumflex +character is an assertion that is true only if the current matching point is at +the start of the subject string. If the startoffset argument of +pcre2_match() is non-zero, circumflex can never match if the +PCRE2_MULTILINE option is unset. Inside a character class, circumflex has an +entirely different meaning +(see below). +
++Circumflex need not be the first character of the pattern if a number of +alternatives are involved, but it should be the first thing in each alternative +in which it appears if the pattern is ever to match that branch. If all +possible alternatives start with a circumflex, that is, if the pattern is +constrained to match only at the start of the subject, it is said to be an +"anchored" pattern. (There are also other constructs that can cause a pattern +to be anchored.) +
++The dollar character is an assertion that is true only if the current matching +point is at the end of the subject string, or immediately before a newline at +the end of the string (by default). Note, however, that it does not actually +match the newline. Dollar need not be the last character of the pattern if a +number of alternatives are involved, but it should be the last item in any +branch in which it appears. Dollar has no special meaning in a character class. +
++The meaning of dollar can be changed so that it matches only at the very end of +the string, by setting the PCRE2_DOLLAR_ENDONLY option at compile time. This +does not affect the \Z assertion. +
++The meanings of the circumflex and dollar characters are changed if the +PCRE2_MULTILINE option is set. When this is the case, a circumflex matches +immediately after internal newlines as well as at the start of the subject +string. It does not match after a newline that ends the string. A dollar +matches before any newlines in the string, as well as at the very end, when +PCRE2_MULTILINE is set. When newline is specified as the two-character +sequence CRLF, isolated CR and LF characters do not indicate newlines. +
++For example, the pattern /^abc$/ matches the subject string "def\nabc" (where +\n represents a newline) in multiline mode, but not otherwise. Consequently, +patterns that are anchored in single line mode because all branches start with +^ are not anchored in multiline mode, and a match for circumflex is possible +when the startoffset argument of pcre2_match() is non-zero. The +PCRE2_DOLLAR_ENDONLY option is ignored if PCRE2_MULTILINE is set. +
++Note that the sequences \A, \Z, and \z can be used to match the start and +end of the subject in both modes, and if all branches of a pattern start with +\A it is always anchored, whether or not PCRE2_MULTILINE is set. +
++Outside a character class, a dot in the pattern matches any one character in +the subject string except (by default) a character that signifies the end of a +line. +
++When a line ending is defined as a single character, dot never matches that +character; when the two-character sequence CRLF is used, dot does not match CR +if it is immediately followed by LF, but otherwise it matches all characters +(including isolated CRs and LFs). When any Unicode line endings are being +recognized, dot does not match CR or LF or any of the other line ending +characters. +
++The behaviour of dot with regard to newlines can be changed. If the +PCRE2_DOTALL option is set, a dot matches any one character, without exception. +If the two-character sequence CRLF is present in the subject string, it takes +two dots to match it. +
++The handling of dot is entirely independent of the handling of circumflex and +dollar, the only relationship being that they both involve newlines. Dot has no +special meaning in a character class. +
++The escape sequence \N behaves like a dot, except that it is not affected by +the PCRE2_DOTALL option. In other words, it matches any character except one +that signifies the end of a line. Perl also uses \N to match characters by +name; PCRE2 does not support this. +
++Outside a character class, the escape sequence \C matches any one code unit, +whether or not a UTF mode is set. In the 8-bit library, one code unit is one +byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is a +32-bit unit. Unlike a dot, \C always matches line-ending characters. The +feature is provided in Perl in order to match individual bytes in UTF-8 mode, +but it is unclear how it can usefully be used. Because \C breaks up characters +into individual code units, matching one unit with \C in a UTF mode means that +the rest of the string may start with a malformed UTF character. This has +undefined results, because PCRE2 assumes that it is dealing with valid UTF +strings (and by default it checks this at the start of processing unless the +PCRE2_NO_UTF_CHECK option is used). +
++PCRE2 does not allow \C to appear in lookbehind assertions +(described below) +in a UTF mode, because this would make it impossible to calculate the length of +the lookbehind. +
++In general, the \C escape sequence is best avoided. However, one way of using +it that avoids the problem of malformed UTF characters is to use a lookahead to +check the length of the next character, as in this pattern, which could be used +with a UTF-8 string (ignore white space and line breaks): +
+ (?| (?=[\x00-\x7f])(\C) |
+ (?=[\x80-\x{7ff}])(\C)(\C) |
+ (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
+ (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
+
+In this example, a group that starts with (?| resets the capturing parentheses
+numbers in each alternative (see
+"Duplicate Subpattern Numbers"
+below). The assertions at the start of each branch check the next UTF-8
+character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
+character's individual bytes are then captured by the appropriate number of
+\C groups.
+
++An opening square bracket introduces a character class, terminated by a closing +square bracket. A closing square bracket on its own is not special by default. +If a closing square bracket is required as a member of the class, it should be +the first data character in the class (after an initial circumflex, if present) +or escaped with a backslash. This means that, by default, an empty class cannot +be defined. However, if the PCRE2_ALLOW_EMPTY_CLASS option is set, a closing +square bracket at the start does end the (empty) class. +
++A character class matches a single character in the subject. A matched +character must be in the set of characters defined by the class, unless the +first character in the class definition is a circumflex, in which case the +subject character must not be in the set defined by the class. If a circumflex +is actually required as a member of the class, ensure it is not the first +character, or escape it with a backslash. +
++For example, the character class [aeiou] matches any lower case vowel, while +[^aeiou] matches any character that is not a lower case vowel. Note that a +circumflex is just a convenient notation for specifying the characters that +are in the class by enumerating those that are not. A class that starts with a +circumflex is not an assertion; it still consumes a character from the subject +string, and therefore it fails if the current pointer is at the end of the +string. +
++When caseless matching is set, any letters in a class represent both their +upper case and lower case versions, so for example, a caseless [aeiou] matches +"A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a +caseful version would. +
++Characters that might indicate line breaks are never treated in any special way +when matching character classes, whatever line-ending sequence is in use, and +whatever setting of the PCRE2_DOTALL and PCRE2_MULTILINE options is used. A +class such as [^a] always matches one of these characters. +
++The minus (hyphen) character can be used to specify a range of characters in a +character class. For example, [d-m] matches any letter between d and m, +inclusive. If a minus character is required in a class, it must be escaped with +a backslash or appear in a position where it cannot be interpreted as +indicating a range, typically as the first or last character in the class, or +immediately after a range. For example, [b-d-z] matches letters in the range b +to d, a hyphen character, or z. +
++It is not possible to have the literal character "]" as the end character of a +range. A pattern such as [W-]46] is interpreted as a class of two characters +("W" and "-") followed by a literal string "46]", so it would match "W46]" or +"-46]". However, if the "]" is escaped with a backslash it is interpreted as +the end of range, so [W-\]46] is interpreted as a class containing a range +followed by two other characters. The octal or hexadecimal representation of +"]" can also be used to end a range. +
++An error is generated if a POSIX character class (see below) or an escape +sequence other than one that defines a single character appears at a point +where a range ending character is expected. For example, [z-\xff] is valid, +but [A-\d] and [A-[:digit:]] are not. +
++Ranges operate in the collating sequence of character values. They can also be +used for characters specified numerically, for example [\000-\037]. Ranges +can include any characters that are valid for the current mode. +
++If a range that includes letters is used when caseless matching is set, it +matches the letters in either case. For example, [W-c] is equivalent to +[][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character +tables for a French locale are in use, [\xc8-\xcb] matches accented E +characters in both cases. +
++The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, +\V, \w, and \W may appear in a character class, and add the characters that +they match to the class. For example, [\dABCDEF] matches any hexadecimal +digit. In UTF modes, the PCRE2_UCP option affects the meanings of \d, \s, \w +and their upper case partners, just as it does when they appear outside a +character class, as described in the section entitled +"Generic character types" +above. The escape sequence \b has a different meaning inside a character +class; it matches the backspace character. The sequences \B, \N, \R, and \X +are not special inside a character class. Like any other unrecognized escape +sequences, they cause an error. +
++A circumflex can conveniently be used with the upper case character types to +specify a more restricted set of characters than the matching lower case type. +For example, the class [^\W_] matches any letter or digit, but not underscore, +whereas [\w] includes underscore. A positive character class should be read as +"something OR something OR ..." and a negative class as "NOT something AND NOT +something AND NOT ...". +
++The only metacharacters that are recognized in character classes are backslash, +hyphen (only where it can be interpreted as specifying a range), circumflex +(only at the start), opening square bracket (only when it can be interpreted as +introducing a POSIX class name, or for a special compatibility feature - see +the next two sections), and the terminating closing square bracket. However, +escaping other non-alphanumeric characters does no harm. +
++Perl supports the POSIX notation for character classes. This uses names +enclosed by [: and :] within the enclosing square brackets. PCRE2 also supports +this notation. For example, +
+ [01[:alpha:]%] ++matches "0", "1", any alphabetic character, or "%". The supported class names +are: +
+ alnum letters and digits + alpha letters + ascii character codes 0 - 127 + blank space or tab only + cntrl control characters + digit decimal digits (same as \d) + graph printing characters, excluding space + lower lower case letters + print printing characters, including space + punct printing characters, excluding letters and digits and space + space white space (the same as \s from PCRE2 8.34) + upper upper case letters + word "word" characters (same as \w) + xdigit hexadecimal digits ++The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), +and space (32). If locale-specific matching is taking place, the list of space +characters may be different; there may be fewer or more of them. "Space" and +\s match the same set of characters. + +
+The name "word" is a Perl extension, and "blank" is a GNU extension from Perl +5.8. Another Perl extension is negation, which is indicated by a ^ character +after the colon. For example, +
+ [12[:^digit:]] ++matches "1", "2", or any non-digit. PCRE2 (and Perl) also recognize the POSIX +syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not +supported, and an error is given if they are encountered. + +
+By default, characters with values greater than 128 do not match any of the +POSIX character classes. However, if the PCRE2_UCP option is passed to +pcre2_compile(), some of the classes are changed so that Unicode +character properties are used. This is achieved by replacing certain POSIX +classes by other sequences, as follows: +
+ [:alnum:] becomes \p{Xan}
+ [:alpha:] becomes \p{L}
+ [:blank:] becomes \h
+ [:cntrl:] becomes \p{Cc}
+ [:digit:] becomes \p{Nd}
+ [:lower:] becomes \p{Ll}
+ [:space:] becomes \p{Xps}
+ [:upper:] becomes \p{Lu}
+ [:word:] becomes \p{Xwd}
+
+Negated versions, such as [:^alpha:] use \P instead of \p. Three other POSIX
+classes are handled specially in UCP mode:
+
++[:graph:] +This matches characters that have glyphs that mark the page when printed. In +Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf +properties, except for: +
+ U+061C Arabic Letter Mark + U+180E Mongolian Vowel Separator + U+2066 - U+2069 Various "isolate"s + ++ +
+[:print:] +This matches the same characters as [:graph:] plus space characters that are +not controls, that is, characters with the Zs property. +
++[:punct:] +This matches all characters that have the Unicode P (punctuation) property, +plus those characters with code points less than 128 that have the S (Symbol) +property. +
++The other POSIX classes are unchanged, and match only characters with code +points less than 128. +
++In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly +syntax [[:<:]] and [[:>:]] is used for matching "start of word" and "end of +word". PCRE2 treats these items as follows: +
+ [[:<:]] is converted to \b(?=\w) + [[:>:]] is converted to \b(?<=\w) ++Only these exact character sequences are recognized. A sequence such as +[a[:<:]b] provokes error for an unrecognized POSIX class name. This support is +not compatible with Perl. It is provided to help migrations from other +environments, and is best not used in any new patterns. Note that \b matches +at the start and the end of a word (see +"Simple assertions" +above), and in a Perl-style pattern the preceding or following character +normally shows which is wanted, without the need for the assertions that are +used above in order to give exactly the POSIX behaviour. + +
+Vertical bar characters are used to separate alternative patterns. For example, +the pattern +
+ gilbert|sullivan ++matches either "gilbert" or "sullivan". Any number of alternatives may appear, +and an empty alternative is permitted (matching the empty string). The matching +process tries each alternative in turn, from left to right, and the first one +that succeeds is used. If the alternatives are within a subpattern +(defined below), +"succeeds" means matching the rest of the main pattern as well as the +alternative in the subpattern. + +
+The settings of the PCRE2_CASELESS, PCRE2_MULTILINE, PCRE2_DOTALL, and +PCRE2_EXTENDED options (which are Perl-compatible) can be changed from within +the pattern by a sequence of Perl option letters enclosed between "(?" and ")". +The option letters are +
+ i for PCRE2_CASELESS + m for PCRE2_MULTILINE + s for PCRE2_DOTALL + x for PCRE2_EXTENDED ++For example, (?im) sets caseless, multiline matching. It is also possible to +unset these options by preceding the letter with a hyphen, and a combined +setting and unsetting such as (?im-sx), which sets PCRE2_CASELESS and +PCRE2_MULTILINE while unsetting PCRE2_DOTALL and PCRE2_EXTENDED, is also +permitted. If a letter appears both before and after the hyphen, the option is +unset. + +
+The PCRE2-specific options PCRE2_DUPNAMES and PCRE2_UNGREEDY can be changed in +the same way as the Perl-compatible options by using the characters J and U +respectively. +
++When one of these option changes occurs at top level (that is, not inside +subpattern parentheses), the change applies to the remainder of the pattern +that follows. If the change is placed right at the start of a pattern, PCRE2 +extracts it into the global options (and it will therefore show up in data +extracted by the pcre2_pattern_info() function). +
++An option change within a subpattern (see below for a description of +subpatterns) affects only that part of the subpattern that follows it, so +
+ (a(?i)b)c ++matches abc and aBc and no other strings (assuming PCRE2_CASELESS is not used). +By this means, options can be made to have different settings in different +parts of the pattern. Any changes made in one alternative do carry on +into subsequent branches within the same subpattern. For example, +
+ (a(?i)b|c) ++matches "ab", "aB", "c", and "C", even though when matching "C" the first +branch is abandoned before the option setting. This is because the effects of +option settings happen at compile time. There would be some very weird +behaviour otherwise. + +
+Note: There are other PCRE2-specific options that can be set by the +application when the compiling function is called. +The pattern can contain special leading sequences such as (*CRLF) to override +what the application has set or what has been defaulted. Details are given in +the section entitled +"Newline sequences" +above. There are also the (*UTF) and (*UCP) leading sequences that can be used +to set UTF and Unicode property modes; they are equivalent to setting the +PCRE2_UTF and PCRE2_UCP options, respectively. However, the application can set +the PCRE2_NEVER_UTF and PCRE2_NEVER_UCP options, which lock out the use of the +(*UTF) and (*UCP) sequences. +
+
+Subpatterns are delimited by parentheses (round brackets), which can be nested.
+Turning part of a pattern into a subpattern does two things:
+
+
+1. It localizes a set of alternatives. For example, the pattern
+
+ cat(aract|erpillar|) ++matches "cataract", "caterpillar", or "cat". Without the parentheses, it would +match "cataract", "erpillar" or an empty string. +
+Opening parentheses are counted from left to right (starting from 1) to obtain +numbers for the capturing subpatterns. For example, if the string "the red +king" is matched against the pattern +
+ the ((red|white) (king|queen)) ++the captured substrings are "red king", "red", and "king", and are numbered 1, +2, and 3, respectively. + +
+The fact that plain parentheses fulfil two functions is not always helpful. +There are often times when a grouping subpattern is required without a +capturing requirement. If an opening parenthesis is followed by a question mark +and a colon, the subpattern does not do any capturing, and is not counted when +computing the number of any subsequent capturing subpatterns. For example, if +the string "the white queen" is matched against the pattern +
+ the ((?:red|white) (king|queen)) ++the captured substrings are "white queen" and "queen", and are numbered 1 and +2. The maximum number of capturing subpatterns is 65535. + +
+As a convenient shorthand, if any option settings are required at the start of +a non-capturing subpattern, the option letters may appear between the "?" and +the ":". Thus the two patterns +
+ (?i:saturday|sunday) + (?:(?i)saturday|sunday) ++match exactly the same set of strings. Because alternative branches are tried +from left to right, and options are not reset until the end of the subpattern +is reached, an option setting in one branch does affect subsequent branches, so +the above patterns match "SUNDAY" as well as "Saturday". + +
+Perl 5.10 introduced a feature whereby each alternative in a subpattern uses +the same numbers for its capturing parentheses. Such a subpattern starts with +(?| and is itself a non-capturing subpattern. For example, consider this +pattern: +
+ (?|(Sat)ur|(Sun))day ++Because the two alternatives are inside a (?| group, both sets of capturing +parentheses are numbered one. Thus, when the pattern matches, you can look +at captured substring number one, whichever alternative matched. This construct +is useful when you want to capture part, but not all, of one of a number of +alternatives. Inside a (?| group, parentheses are numbered as usual, but the +number is reset at the start of each branch. The numbers of any capturing +parentheses that follow the subpattern start after the highest number used in +any branch. The following example is taken from the Perl documentation. The +numbers underneath show in which buffer the captured content will be stored. +
+ # before ---------------branch-reset----------- after + / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x + # 1 2 2 3 2 3 4 ++A back reference to a numbered subpattern uses the most recent value that is +set for that number by any subpattern. The following pattern matches "abcabc" +or "defdef": +
+ /(?|(abc)|(def))\1/ ++In contrast, a subroutine call to a numbered subpattern always refers to the +first one in the pattern with the given number. The following pattern matches +"abcabc" or "defabc": +
+ /(?|(abc)|(def))(?1)/ ++If a +condition test +for a subpattern's having matched refers to a non-unique number, the test is +true if any of the subpatterns of that number have matched. + +
+An alternative approach to using this "branch reset" feature is to use +duplicate named subpatterns, as described in the next section. +
++Identifying capturing parentheses by number is simple, but it can be very hard +to keep track of the numbers in complicated regular expressions. Furthermore, +if an expression is modified, the numbers may change. To help with this +difficulty, PCRE2 supports the naming of subpatterns. This feature was not +added to Perl until release 5.10. Python had the feature earlier, and PCRE1 +introduced it at release 4.0, using the Python syntax. PCRE2 supports both the +Perl and the Python syntax. Perl allows identically numbered subpatterns to +have different names, but PCRE2 does not. +
++In PCRE2, a subpattern can be named in one of three ways: (?<name>...) or +(?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing +parentheses from other parts of the pattern, such as +back references, +recursion, +and +conditions, +can be made by name as well as by number. +
++Names consist of up to 32 alphanumeric characters and underscores, but must +start with a non-digit. Named capturing parentheses are still allocated numbers +as well as names, exactly as if the names were not present. The PCRE2 API +provides function calls for extracting the name-to-number translation table +from a compiled pattern. There are also convenience functions for extracting a +captured substring by name. +
++By default, a name must be unique within a pattern, but it is possible to relax +this constraint by setting the PCRE2_DUPNAMES option at compile time. +(Duplicate names are also always permitted for subpatterns with the same +number, set up as described in the previous section.) Duplicate names can be +useful for patterns where only one instance of the named parentheses can match. +Suppose you want to match the name of a weekday, either as a 3-letter +abbreviation or as the full name, and in both cases you want to extract the +abbreviation. This pattern (ignoring the line breaks) does the job: +
+ (?<DN>Mon|Fri|Sun)(?:day)?| + (?<DN>Tue)(?:sday)?| + (?<DN>Wed)(?:nesday)?| + (?<DN>Thu)(?:rsday)?| + (?<DN>Sat)(?:urday)? ++There are five capturing substrings, but only one is ever set after a match. +(An alternative way of solving this problem is to use a "branch reset" +subpattern, as described in the previous section.) + +
+The convenience functions for extracting the data by name returns the substring +for the first (and in this example, the only) subpattern of that name that +matched. This saves searching to find which numbered subpattern it was. +
++If you make a back reference to a non-unique named subpattern from elsewhere in +the pattern, the subpatterns to which the name refers are checked in the order +in which they appear in the overall pattern. The first one that is set is used +for the reference. For example, this pattern matches both "foofoo" and +"barbar" but not "foobar" or "barfoo": +
+ (?:(?<n>foo)|(?<n>bar))\k<n> + ++ +
+If you make a subroutine call to a non-unique named subpattern, the one that +corresponds to the first occurrence of the name is used. In the absence of +duplicate numbers (see the previous section) this is the one with the lowest +number. +
++If you use a named reference in a condition +test (see the +section about conditions +below), either to check whether a subpattern has matched, or to check for +recursion, all subpatterns with the same name are tested. If the condition is +true for any one of them, the overall condition is true. This is the same +behaviour as testing by number. For further details of the interfaces for +handling named subpatterns, see the +pcre2api +documentation. +
++Warning: You cannot use different names to distinguish between two +subpatterns with the same number because PCRE2 uses only the numbers when +matching. For this reason, an error is given at compile time if different names +are given to subpatterns with the same number. However, you can always give the +same name to subpatterns with the same number, even when PCRE2_DUPNAMES is not +set. +
++Repetition is specified by quantifiers, which can follow any of the following +items: +
+ a literal data character + the dot metacharacter + the \C escape sequence + the \X escape sequence + the \R escape sequence + an escape such as \d or \pL that matches a single character + a character class + a back reference (see next section) + a parenthesized subpattern (including assertions) + a subroutine call to a subpattern (recursive or otherwise) ++The general repetition quantifier specifies a minimum and maximum number of +permitted matches, by giving the two numbers in curly brackets (braces), +separated by a comma. The numbers must be less than 65536, and the first must +be less than or equal to the second. For example: +
+ z{2,4}
+
+matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
+character. If the second number is omitted, but the comma is present, there is
+no upper limit; if the second number and the comma are both omitted, the
+quantifier specifies an exact number of required matches. Thus
+
+ [aeiou]{3,}
+
+matches at least 3 successive vowels, but may match many more, whereas
+
+ \d{8}
+
+matches exactly 8 digits. An opening curly bracket that appears in a position
+where a quantifier is not allowed, or one that does not match the syntax of a
+quantifier, is taken as a literal character. For example, {,6} is not a
+quantifier, but a literal string of four characters.
+
++In UTF modes, quantifiers apply to characters rather than to individual code +units. Thus, for example, \x{100}{2} matches two characters, each of +which is represented by a two-byte sequence in a UTF-8 string. Similarly, +\X{3} matches three Unicode extended grapheme clusters, each of which may be +several code units long (and they may be of different lengths). +
++The quantifier {0} is permitted, causing the expression to behave as if the +previous item and the quantifier were not present. This may be useful for +subpatterns that are referenced as +subroutines +from elsewhere in the pattern (but see also the section entitled +"Defining subpatterns for use by reference only" +below). Items other than subpatterns that have a {0} quantifier are omitted +from the compiled pattern. +
++For convenience, the three most common quantifiers have single-character +abbreviations: +
+ * is equivalent to {0,}
+ + is equivalent to {1,}
+ ? is equivalent to {0,1}
+
+It is possible to construct infinite loops by following a subpattern that can
+match no characters with a quantifier that has no upper limit, for example:
++ (a?)* ++Earlier versions of Perl and PCRE1 used to give an error at compile time for +such patterns. However, because there are cases where this can be useful, such +patterns are now accepted, but if any repetition of the subpattern does in fact +match no characters, the loop is forcibly broken. + +
+By default, the quantifiers are "greedy", that is, they match as much as +possible (up to the maximum number of permitted times), without causing the +rest of the pattern to fail. The classic example of where this gives problems +is in trying to match comments in C programs. These appear between /* and */ +and within the comment, individual * and / characters may appear. An attempt to +match C comments by applying the pattern +
+ /\*.*\*/ ++to the string +
+ /* first comment */ not comment /* second comment */ ++fails, because it matches the entire string owing to the greediness of the .* +item. + +
+If a quantifier is followed by a question mark, it ceases to be greedy, and +instead matches the minimum number of times possible, so the pattern +
+ /\*.*?\*/ ++does the right thing with the C comments. The meaning of the various +quantifiers is not otherwise changed, just the preferred number of matches. +Do not confuse this use of question mark with its use as a quantifier in its +own right. Because it has two uses, it can sometimes appear doubled, as in +
+ \d??\d ++which matches one digit by preference, but can match two if that is the only +way the rest of the pattern matches. + +
+If the PCRE2_UNGREEDY option is set (an option that is not available in Perl), +the quantifiers are not greedy by default, but individual ones can be made +greedy by following them with a question mark. In other words, it inverts the +default behaviour. +
++When a parenthesized subpattern is quantified with a minimum repeat count that +is greater than 1 or with a limited maximum, more memory is required for the +compiled pattern, in proportion to the size of the minimum or maximum. +
++If a pattern starts with .* or .{0,} and the PCRE2_DOTALL option (equivalent +to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is +implicitly anchored, because whatever follows will be tried against every +character position in the subject string, so there is no point in retrying the +overall match at any position after the first. PCRE2 normally treats such a +pattern as though it were preceded by \A. +
++In cases where it is known that the subject string contains no newlines, it is +worth setting PCRE2_DOTALL in order to obtain this optimization, or +alternatively, using ^ to indicate anchoring explicitly. +
++However, there are some cases where the optimization cannot be used. When .* +is inside capturing parentheses that are the subject of a back reference +elsewhere in the pattern, a match at the start may fail where a later one +succeeds. Consider, for example: +
+ (.*)abc\1 ++If the subject is "xyz123abc123" the match point is the fourth character. For +this reason, such a pattern is not implicitly anchored. + +
+Another case where implicit anchoring is not applied is when the leading .* is +inside an atomic group. Once again, a match at the start may fail where a later +one succeeds. Consider this pattern: +
+ (?>.*?a)b ++It matches "ab" in the subject "aab". The use of the backtracking control verbs +(*PRUNE) and (*SKIP) also disable this optimization. + +
+When a capturing subpattern is repeated, the value captured is the substring +that matched the final iteration. For example, after +
+ (tweedle[dume]{3}\s*)+
+
+has matched "tweedledum tweedledee" the value of the captured substring is
+"tweedledee". However, if there are nested capturing subpatterns, the
+corresponding captured values may have been set in previous iterations. For
+example, after
++ (a|(b))+ ++matches "aba" the value of the second captured substring is "b". + +
+With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") +repetition, failure of what follows normally causes the repeated item to be +re-evaluated to see if a different number of repeats allows the rest of the +pattern to match. Sometimes it is useful to prevent this, either to change the +nature of the match, or to cause it fail earlier than it otherwise might, when +the author of the pattern knows there is no point in carrying on. +
++Consider, for example, the pattern \d+foo when applied to the subject line +
+ 123456bar ++After matching all 6 digits and then failing to match "foo", the normal +action of the matcher is to try again with only 5 digits matching the \d+ +item, and then with 4, and so on, before ultimately failing. "Atomic grouping" +(a term taken from Jeffrey Friedl's book) provides the means for specifying +that once a subpattern has matched, it is not to be re-evaluated in this way. + +
+If we use atomic grouping for the previous example, the matcher gives up +immediately on failing to match "foo" the first time. The notation is a kind of +special parenthesis, starting with (?> as in this example: +
+ (?>\d+)foo ++This kind of parenthesis "locks up" the part of the pattern it contains once +it has matched, and a failure further into the pattern is prevented from +backtracking into it. Backtracking past it to previous items, however, works as +normal. + +
+An alternative description is that a subpattern of this type matches exactly +the string of characters that an identical standalone pattern would match, if +anchored at the current point in the subject string. +
++Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as +the above example can be thought of as a maximizing repeat that must swallow +everything it can. So, while both \d+ and \d+? are prepared to adjust the +number of digits they match in order to make the rest of the pattern match, +(?>\d+) can only match an entire sequence of digits. +
++Atomic groups in general can of course contain arbitrarily complicated +subpatterns, and can be nested. However, when the subpattern for an atomic +group is just a single repeated item, as in the example above, a simpler +notation, called a "possessive quantifier" can be used. This consists of an +additional + character following a quantifier. Using this notation, the +previous example can be rewritten as +
+ \d++foo ++Note that a possessive quantifier can be used with an entire group, for +example: +
+ (abc|xyz){2,3}+
+
+Possessive quantifiers are always greedy; the setting of the PCRE2_UNGREEDY
+option is ignored. They are a convenient notation for the simpler forms of
+atomic group. However, there is no difference in the meaning of a possessive
+quantifier and the equivalent atomic group, though there may be a performance
+difference; possessive quantifiers should be slightly faster.
+
++The possessive quantifier syntax is an extension to the Perl 5.8 syntax. +Jeffrey Friedl originated the idea (and the name) in the first edition of his +book. Mike McCloskey liked it, so implemented it when he built Sun's Java +package, and PCRE1 copied it from there. It ultimately found its way into Perl +at release 5.10. +
++PCRE2 has an optimization that automatically "possessifies" certain simple +pattern constructs. For example, the sequence A+B is treated as A++B because +there is no point in backtracking into a sequence of A's when B must follow. +This feature can be disabled by the PCRE2_NO_AUTOPOSSESS option, or starting +the pattern with (*NO_AUTO_POSSESS). +
++When a pattern contains an unlimited repeat inside a subpattern that can itself +be repeated an unlimited number of times, the use of an atomic group is the +only way to avoid some failing matches taking a very long time indeed. The +pattern +
+ (\D+|<\d+>)*[!?] ++matches an unlimited number of substrings that either consist of non-digits, or +digits enclosed in <>, followed by either ! or ?. When it matches, it runs +quickly. However, if it is applied to +
+ aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa ++it takes a long time before reporting failure. This is because the string can +be divided between the internal \D+ repeat and the external * repeat in a +large number of ways, and all have to be tried. (The example uses [!?] rather +than a single character at the end, because both PCRE2 and Perl have an +optimization that allows for fast failure when a single character is used. They +remember the last single character that is required for a match, and fail early +if it is not present in the string.) If the pattern is changed so that it uses +an atomic group, like this: +
+ ((?>\D+)|<\d+>)*[!?] ++sequences of non-digits cannot be broken, and failure happens quickly. + +
+Outside a character class, a backslash followed by a digit greater than 0 (and +possibly further digits) is a back reference to a capturing subpattern earlier +(that is, to its left) in the pattern, provided there have been that many +previous capturing left parentheses. +
++However, if the decimal number following the backslash is less than 8, it is +always taken as a back reference, and causes an error only if there are not +that many capturing left parentheses in the entire pattern. In other words, the +parentheses that are referenced need not be to the left of the reference for +numbers less than 8. A "forward back reference" of this type can make sense +when a repetition is involved and the subpattern to the right has participated +in an earlier iteration. +
++It is not possible to have a numerical "forward back reference" to a subpattern +whose number is 8 or more using this syntax because a sequence such as \50 is +interpreted as a character defined in octal. See the subsection entitled +"Non-printing characters" +above +for further details of the handling of digits following a backslash. There is +no such problem when named parentheses are used. A back reference to any +subpattern is possible using named parentheses (see below). +
++Another way of avoiding the ambiguity inherent in the use of digits following a +backslash is to use the \g escape sequence. This escape must be followed by an +unsigned number or a negative number, optionally enclosed in braces. These +examples are all identical: +
+ (ring), \1
+ (ring), \g1
+ (ring), \g{1}
+
+An unsigned number specifies an absolute reference without the ambiguity that
+is present in the older syntax. It is also useful when literal digits follow
+the reference. A negative number is a relative reference. Consider this
+example:
+
+ (abc(def)ghi)\g{-1}
+
+The sequence \g{-1} is a reference to the most recently started capturing
+subpattern before \g, that is, is it equivalent to \2 in this example.
+Similarly, \g{-2} would be equivalent to \1. The use of relative references
+can be helpful in long patterns, and also in patterns that are created by
+joining together fragments that contain references within themselves.
+
++A back reference matches whatever actually matched the capturing subpattern in +the current subject string, rather than anything matching the subpattern +itself (see +"Subpatterns as subroutines" +below for a way of doing that). So the pattern +
+ (sens|respons)e and \1ibility ++matches "sense and sensibility" and "response and responsibility", but not +"sense and responsibility". If caseful matching is in force at the time of the +back reference, the case of letters is relevant. For example, +
+ ((?i)rah)\s+\1 ++matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original +capturing subpattern is matched caselessly. + +
+There are several different ways of writing back references to named +subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or +\k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified +back reference syntax, in which \g can be used for both numeric and named +references, is also supported. We could rewrite the above example in any of +the following ways: +
+ (?<p1>(?i)rah)\s+\k<p1>
+ (?'p1'(?i)rah)\s+\k{p1}
+ (?P<p1>(?i)rah)\s+(?P=p1)
+ (?<p1>(?i)rah)\s+\g{p1}
+
+A subpattern that is referenced by name may appear in the pattern before or
+after the reference.
+
++There may be more than one back reference to the same subpattern. If a +subpattern has not actually been used in a particular match, any back +references to it always fail by default. For example, the pattern +
+ (a|(bc))\2 ++always fails if it starts to match "a" rather than "bc". However, if the +PCRE2_MATCH_UNSET_BACKREF option is set at compile time, a back reference to an +unset value matches an empty string. + +
+Because there may be many capturing parentheses in a pattern, all digits +following a backslash are taken as part of a potential back reference number. +If the pattern continues with a digit character, some delimiter must be used to +terminate the back reference. If the PCRE2_EXTENDED option is set, this can be +white space. Otherwise, the \g{ syntax or an empty comment (see +"Comments" +below) can be used. +
++A back reference that occurs inside the parentheses to which it refers fails +when the subpattern is first used, so, for example, (a\1) never matches. +However, such references can be useful inside repeated subpatterns. For +example, the pattern +
+ (a|b\1)+ ++matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of +the subpattern, the back reference matches the character string corresponding +to the previous iteration. In order for this to work, the pattern must be such +that the first iteration does not need to match the back reference. This can be +done using alternation, as in the example above, or by a quantifier with a +minimum of zero. + +
+Back references of this type cause the group that they reference to be treated +as an +atomic group. +Once the whole group has been matched, a subsequent matching failure cannot +cause backtracking into the middle of the group. +
++An assertion is a test on the characters following or preceding the current +matching point that does not consume any characters. The simple assertions +coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described +above. +
++More complicated assertions are coded as subpatterns. There are two kinds: +those that look ahead of the current position in the subject string, and those +that look behind it. An assertion subpattern is matched in the normal way, +except that it does not cause the current matching position to be changed. +
++Assertion subpatterns are not capturing subpatterns. If such an assertion +contains capturing subpatterns within it, these are counted for the purposes of +numbering the capturing subpatterns in the whole pattern. However, substring +capturing is carried out only for positive assertions. (Perl sometimes, but not +always, does do capturing in negative assertions.) +
+
+For compatibility with Perl, assertion subpatterns may be repeated; though
+it makes no sense to assert the same thing several times, the side effect of
+capturing parentheses may occasionally be useful. In practice, there only three
+cases:
+
+
+(1) If the quantifier is {0}, the assertion is never obeyed during matching.
+However, it may contain internal capturing parenthesized groups that are called
+from elsewhere via the
+subroutine mechanism.
+
+
+(2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
+were {0,1}. At run time, the rest of the pattern match is tried with and
+without the assertion, the order depending on the greediness of the quantifier.
+
+
+(3) If the minimum repetition is greater than zero, the quantifier is ignored.
+The assertion is obeyed just once when encountered during matching.
+
+Lookahead assertions start with (?= for positive assertions and (?! for +negative assertions. For example, +
+ \w+(?=;) ++matches a word followed by a semicolon, but does not include the semicolon in +the match, and +
+ foo(?!bar) ++matches any occurrence of "foo" that is not followed by "bar". Note that the +apparently similar pattern +
+ (?!foo)bar ++does not find an occurrence of "bar" that is preceded by something other than +"foo"; it finds any occurrence of "bar" whatsoever, because the assertion +(?!foo) is always true when the next three characters are "bar". A +lookbehind assertion is needed to achieve the other effect. + +
+If you want to force a matching failure at some point in a pattern, the most +convenient way to do it is with (?!) because an empty string always matches, so +an assertion that requires there not to be an empty string must always fail. +The backtracking control verb (*FAIL) or (*F) is a synonym for (?!). +
++Lookbehind assertions start with (?<= for positive assertions and (?<! for +negative assertions. For example, +
+ (?<!foo)bar ++does find an occurrence of "bar" that is not preceded by "foo". The contents of +a lookbehind assertion are restricted such that all the strings it matches must +have a fixed length. However, if there are several top-level alternatives, they +do not all have to have the same fixed length. Thus +
+ (?<=bullock|donkey) ++is permitted, but +
+ (?<!dogs?|cats?) ++causes an error at compile time. Branches that match different length strings +are permitted only at the top level of a lookbehind assertion. This is an +extension compared with Perl, which requires all branches to match the same +length of string. An assertion such as +
+ (?<=ab(c|de)) ++is not permitted, because its single top-level branch can match two different +lengths, but it is acceptable to PCRE2 if rewritten to use two top-level +branches: +
+ (?<=abc|abde) ++In some cases, the escape sequence \K +(see above) +can be used instead of a lookbehind assertion to get round the fixed-length +restriction. + +
+The implementation of lookbehind assertions is, for each alternative, to +temporarily move the current position back by the fixed length and then try to +match. If there are insufficient characters before the current position, the +assertion fails. +
++In a UTF mode, PCRE2 does not allow the \C escape (which matches a single code +unit even in a UTF mode) to appear in lookbehind assertions, because it makes +it impossible to calculate the length of the lookbehind. The \X and \R +escapes, which can match different numbers of code units, are also not +permitted. +
++"Subroutine" +calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long +as the subpattern matches a fixed-length string. +Recursion, +however, is not supported. +
++Possessive quantifiers can be used in conjunction with lookbehind assertions to +specify efficient matching of fixed-length strings at the end of subject +strings. Consider a simple pattern such as +
+ abcd$ ++when applied to a long string that does not match. Because matching proceeds +from left to right, PCRE2 will look for each "a" in the subject and then see if +what follows matches the rest of the pattern. If the pattern is specified as +
+ ^.*abcd$ ++the initial .* matches the entire string at first, but when this fails (because +there is no following "a"), it backtracks to match all but the last character, +then all but the last two characters, and so on. Once again the search for "a" +covers the entire string, from right to left, so we are no better off. However, +if the pattern is written as +
+ ^.*+(?<=abcd) ++there can be no backtracking for the .*+ item because of the possessive +quantifier; it can match only the entire string. The subsequent lookbehind +assertion does a single test on the last four characters. If it fails, the +match fails immediately. For long strings, this approach makes a significant +difference to the processing time. + +
+Several assertions (of any sort) may occur in succession. For example, +
+ (?<=\d{3})(?<!999)foo
+
+matches "foo" preceded by three digits that are not "999". Notice that each of
+the assertions is applied independently at the same point in the subject
+string. First there is a check that the previous three characters are all
+digits, and then there is a check that the same three characters are not "999".
+This pattern does not match "foo" preceded by six characters, the first
+of which are digits and the last three of which are not "999". For example, it
+doesn't match "123abcfoo". A pattern to do that is
+
+ (?<=\d{3}...)(?<!999)foo
+
+This time the first assertion looks at the preceding six characters, checking
+that the first three are digits, and then the second assertion checks that the
+preceding three characters are not "999".
+
++Assertions can be nested in any combination. For example, +
+ (?<=(?<!foo)bar)baz ++matches an occurrence of "baz" that is preceded by "bar" which in turn is not +preceded by "foo", while +
+ (?<=\d{3}(?!999)...)foo
+
+is another pattern that matches "foo" preceded by three digits and any three
+characters that are not "999".
+
++It is possible to cause the matching process to obey a subpattern +conditionally or to choose between two alternative subpatterns, depending on +the result of an assertion, or whether a specific capturing subpattern has +already been matched. The two possible forms of conditional subpattern are: +
+ (?(condition)yes-pattern) + (?(condition)yes-pattern|no-pattern) ++If the condition is satisfied, the yes-pattern is used; otherwise the +no-pattern (if present) is used. If there are more than two alternatives in the +subpattern, a compile-time error occurs. Each of the two alternatives may +itself contain nested subpatterns of any form, including conditional +subpatterns; the restriction to two alternatives applies only at the level of +the condition. This pattern fragment is an example where the alternatives are +complex: +
+ (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) + ++ +
+There are four kinds of condition: references to subpatterns, references to +recursion, a pseudo-condition called DEFINE, and assertions. +
++If the text between the parentheses consists of a sequence of digits, the +condition is true if a capturing subpattern of that number has previously +matched. If there is more than one capturing subpattern with the same number +(see the earlier +section about duplicate subpattern numbers), +the condition is true if any of them have matched. An alternative notation is +to precede the digits with a plus or minus sign. In this case, the subpattern +number is relative rather than absolute. The most recently opened parentheses +can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside +loops it can also make sense to refer to subsequent groups. The next +parentheses to be opened can be referenced as (?(+1), and so on. (The value +zero in any of these forms is not used; it provokes a compile-time error.) +
++Consider the following pattern, which contains non-significant white space to +make it more readable (assume the PCRE2_EXTENDED option) and to divide it into +three parts for ease of discussion: +
+ ( \( )? [^()]+ (?(1) \) ) ++The first part matches an optional opening parenthesis, and if that +character is present, sets it as the first captured substring. The second part +matches one or more characters that are not parentheses. The third part is a +conditional subpattern that tests whether or not the first set of parentheses +matched. If they did, that is, if subject started with an opening parenthesis, +the condition is true, and so the yes-pattern is executed and a closing +parenthesis is required. Otherwise, since no-pattern is not present, the +subpattern matches nothing. In other words, this pattern matches a sequence of +non-parentheses, optionally enclosed in parentheses. + +
+If you were embedding this pattern in a larger one, you could use a relative +reference: +
+ ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ... ++This makes the fragment independent of the parentheses in the larger pattern. + +
+Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used +subpattern by name. For compatibility with earlier versions of PCRE1, which had +this facility before Perl, the syntax (?(name)...) is also recognized. +
++Rewriting the above example to use a named subpattern gives this: +
+ (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) ) ++If the name used in a condition of this kind is a duplicate, the test is +applied to all subpatterns of the same name, and is true if any one of them has +matched. + +
+If the condition is the string (R), and there is no subpattern with the name R, +the condition is true if a recursive call to the whole pattern or any +subpattern has been made. If digits or a name preceded by ampersand follow the +letter R, for example: +
+ (?(R3)...) or (?(R&name)...) ++the condition is true if the most recent recursion is into a subpattern whose +number or name is given. This condition does not check the entire recursion +stack. If the name used in a condition of this kind is a duplicate, the test is +applied to all subpatterns of the same name, and is true if any one of them is +the most recent recursion. + +
+At "top level", all these recursion test conditions are false. +The syntax for recursive patterns +is described below. +
++If the condition is the string (DEFINE), and there is no subpattern with the +name DEFINE, the condition is always false. In this case, there may be only one +alternative in the subpattern. It is always skipped if control reaches this +point in the pattern; the idea of DEFINE is that it can be used to define +subroutines that can be referenced from elsewhere. (The use of +subroutines +is described below.) For example, a pattern to match an IPv4 address such as +"192.168.23.245" could be written like this (ignore white space and line +breaks): +
+ (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
+ \b (?&byte) (\.(?&byte)){3} \b
+
+The first part of the pattern is a DEFINE group inside which a another group
+named "byte" is defined. This matches an individual component of an IPv4
+address (a number less than 256). When matching takes place, this part of the
+pattern is skipped because DEFINE acts like a false condition. The rest of the
+pattern uses references to the named group to match the four dot-separated
+components of an IPv4 address, insisting on a word boundary at each end.
+
++If the condition is not in any of the above formats, it must be an assertion. +This may be a positive or negative lookahead or lookbehind assertion. Consider +this pattern, again containing non-significant white space, and with the two +alternatives on the second line: +
+ (?(?=[^a-z]*[a-z])
+ \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
+
+The condition is a positive lookahead assertion that matches an optional
+sequence of non-letters followed by a letter. In other words, it tests for the
+presence of at least one letter in the subject. If a letter is found, the
+subject is matched against the first alternative; otherwise it is matched
+against the second. This pattern matches strings in one of the two forms
+dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
+
++There are two ways of including comments in patterns that are processed by +PCRE2. In both cases, the start of the comment must not be in a character +class, nor in the middle of any other sequence of related characters such as +(?: or a subpattern name or number. The characters that make up a comment play +no part in the pattern matching. +
++The sequence (?# marks the start of a comment that continues up to the next +closing parenthesis. Nested parentheses are not permitted. If the +PCRE2_EXTENDED option is set, an unescaped # character also introduces a +comment, which in this case continues to immediately after the next newline +character or character sequence in the pattern. Which characters are +interpreted as newlines is controlled by an option passed to the compiling +function or by a special sequence at the start of the pattern, as described in +the section entitled +"Newline conventions" +above. Note that the end of this type of comment is a literal newline sequence +in the pattern; escape sequences that happen to represent a newline do not +count. For example, consider this pattern when PCRE2_EXTENDED is set, and the +default newline convention (a single linefeed character) is in force: +
+ abc #comment \n still comment ++On encountering the # character, pcre2_compile() skips along, looking for +a newline in the pattern. The sequence \n is still literal at this stage, so +it does not terminate the comment. Only an actual character with the code value +0x0a (the default newline) does so. + +
+Consider the problem of matching a string in parentheses, allowing for +unlimited nested parentheses. Without the use of recursion, the best that can +be done is to use a pattern that matches up to some fixed depth of nesting. It +is not possible to handle an arbitrary nesting depth. +
++For some time, Perl has provided a facility that allows regular expressions to +recurse (amongst other things). It does this by interpolating Perl code in the +expression at run time, and the code can refer to the expression itself. A Perl +pattern using code interpolation to solve the parentheses problem can be +created like this: +
+ $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
+
+The (?p{...}) item interpolates Perl code at run time, and in this case refers
+recursively to the pattern in which it appears.
+
++Obviously, PCRE2 cannot support the interpolation of Perl code. Instead, it +supports special syntax for recursion of the entire pattern, and also for +individual subpattern recursion. After its introduction in PCRE1 and Python, +this kind of recursion was subsequently introduced into Perl at release 5.10. +
++A special item that consists of (? followed by a number greater than zero and a +closing parenthesis is a recursive subroutine call of the subpattern of the +given number, provided that it occurs inside that subpattern. (If not, it is a +non-recursive subroutine +call, which is described in the next section.) The special item (?R) or (?0) is +a recursive call of the entire regular expression. +
++This PCRE2 pattern solves the nested parentheses problem (assume the +PCRE2_EXTENDED option is set so that white space is ignored): +
+ \( ( [^()]++ | (?R) )* \) ++First it matches an opening parenthesis. Then it matches any number of +substrings which can either be a sequence of non-parentheses, or a recursive +match of the pattern itself (that is, a correctly parenthesized substring). +Finally there is a closing parenthesis. Note the use of a possessive quantifier +to avoid backtracking into sequences of non-parentheses. + +
+If this were part of a larger pattern, you would not want to recurse the entire +pattern, so instead you could use this: +
+ ( \( ( [^()]++ | (?1) )* \) ) ++We have put the pattern into parentheses, and caused the recursion to refer to +them instead of the whole pattern. + +
+In a larger pattern, keeping track of parenthesis numbers can be tricky. This +is made easier by the use of relative references. Instead of (?1) in the +pattern above you can write (?-2) to refer to the second most recently opened +parentheses preceding the recursion. In other words, a negative number counts +capturing parentheses leftwards from the point at which it is encountered. +
++It is also possible to refer to subsequently opened parentheses, by writing +references such as (?+2). However, these cannot be recursive because the +reference is not inside the parentheses that are referenced. They are always +non-recursive subroutine +calls, as described in the next section. +
++An alternative approach is to use named parentheses. The Perl syntax for this +is (?&name); PCRE1's earlier syntax (?P>name) is also supported. We could +rewrite the above example as follows: +
+ (?<pn> \( ( [^()]++ | (?&pn) )* \) ) ++If there is more than one subpattern with the same name, the earliest one is +used. + +
+The example pattern that we have been looking at contains nested unlimited +repeats, and so the use of a possessive quantifier for matching strings of +non-parentheses is important when applying the pattern to strings that do not +match. For example, when this pattern is applied to +
+ (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() ++it yields "no match" quickly. However, if a possessive quantifier is not used, +the match runs for a very long time indeed because there are so many different +ways the + and * repeats can carve up the subject, and all have to be tested +before failure can be reported. + +
+At the end of a match, the values of capturing parentheses are those from +the outermost level. If you want to obtain intermediate values, a callout +function can be used (see below and the +pcre2callout +documentation). If the pattern above is matched against +
+ (ab(cd)ef) ++the value for the inner capturing parentheses (numbered 2) is "ef", which is +the last value taken on at the top level. If a capturing subpattern is not +matched at the top level, its final captured value is unset, even if it was +(temporarily) set at a deeper level during the matching process. + +
+If there are more than 15 capturing parentheses in a pattern, PCRE2 has to +obtain extra memory from the heap to store data during a recursion. If no +memory can be obtained, the match fails with the PCRE2_ERROR_NOMEMORY error. +
++Do not confuse the (?R) item with the condition (R), which tests for recursion. +Consider this pattern, which matches text in angle brackets, allowing for +arbitrary nesting. Only digits are allowed in nested brackets (that is, when +recursing), whereas any characters are permitted at the outer level. +
+ < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > ++In this pattern, (?(R) is the start of a conditional subpattern, with two +different alternatives for the recursive and non-recursive cases. The (?R) item +is the actual recursive call. + +
+Recursion processing in PCRE2 differs from Perl in two important ways. In PCRE2 +(like Python, but unlike Perl), a recursive subpattern call is always treated +as an atomic group. That is, once it has matched some of the subject string, it +is never re-entered, even if it contains untried alternatives and there is a +subsequent matching failure. This can be illustrated by the following pattern, +which purports to match a palindromic string that contains an odd number of +characters (for example, "a", "aba", "abcba", "abcdcba"): +
+ ^(.|(.)(?1)\2)$ ++The idea is that it either matches a single character, or two identical +characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE2 +it does not if the pattern is longer than three characters. Consider the +subject string "abcba": + +
+At the top level, the first character is matched, but as it is not at the end +of the string, the first alternative fails; the second alternative is taken +and the recursion kicks in. The recursive call to subpattern 1 successfully +matches the next character ("b"). (Note that the beginning and end of line +tests are not part of the recursion). +
++Back at the top level, the next character ("c") is compared with what +subpattern 2 matched, which was "a". This fails. Because the recursion is +treated as an atomic group, there are now no backtracking points, and so the +entire match fails. (Perl is able, at this point, to re-enter the recursion and +try the second alternative.) However, if the pattern is written with the +alternatives in the other order, things are different: +
+ ^((.)(?1)\2|.)$ ++This time, the recursing alternative is tried first, and continues to recurse +until it runs out of characters, at which point the recursion fails. But this +time we do have another alternative to try at the higher level. That is the big +difference: in the previous case the remaining alternative is at a deeper +recursion level, which PCRE2 cannot use. + +
+To change the pattern so that it matches all palindromic strings, not just +those with an odd number of characters, it is tempting to change the pattern to +this: +
+ ^((.)(?1)\2|.?)$ ++Again, this works in Perl, but not in PCRE2, and for the same reason. When a +deeper recursion has matched a single character, it cannot be entered again in +order to match an empty string. The solution is to separate the two cases, and +write out the odd and even cases as alternatives at the higher level: +
+ ^(?:((.)(?1)\2|)|((.)(?3)\4|.)) ++If you want to match typical palindromic phrases, the pattern has to ignore all +non-word characters, which can be done like this: +
+ ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$ ++If run with the PCRE2_CASELESS option, this pattern matches phrases such as "A +man, a plan, a canal: Panama!" and it works in both PCRE2 and Perl. Note the +use of the possessive quantifier *+ to avoid backtracking into sequences of +non-word characters. Without this, PCRE2 takes a great deal longer (ten times +or more) to match typical phrases, and Perl takes so long that you think it has +gone into a loop. + +
+WARNING: The palindrome-matching patterns above work only if the subject +string does not start with a palindrome that is shorter than the entire string. +For example, although "abcba" is correctly matched, if the subject is "ababa", +PCRE2 finds the palindrome "aba" at the start, then fails at top level because +the end of the string does not follow. Once again, it cannot jump back into the +recursion to try other alternatives, so the entire match fails. +
++The second way in which PCRE2 and Perl differ in their recursion processing is +in the handling of captured values. In Perl, when a subpattern is called +recursively or as a subpattern (see the next section), it has no access to any +values that were captured outside the recursion, whereas in PCRE2 these values +can be referenced. Consider this pattern: +
+ ^(.)(\1|a(?2)) ++In PCRE2, this pattern matches "bab". The first capturing parentheses match "b", +then in the second group, when the back reference \1 fails to match "b", the +second alternative matches "a" and then recurses. In the recursion, \1 does +now match "b" and so the whole match succeeds. In Perl, the pattern fails to +match because inside the recursive call \1 cannot access the externally set +value. + +
+If the syntax for a recursive subpattern call (either by number or by +name) is used outside the parentheses to which it refers, it operates like a +subroutine in a programming language. The called subpattern may be defined +before or after the reference. A numbered reference can be absolute or +relative, as in these examples: +
+ (...(absolute)...)...(?2)... + (...(relative)...)...(?-1)... + (...(?+1)...(relative)... ++An earlier example pointed out that the pattern +
+ (sens|respons)e and \1ibility ++matches "sense and sensibility" and "response and responsibility", but not +"sense and responsibility". If instead the pattern +
+ (sens|respons)e and (?1)ibility ++is used, it does match "sense and responsibility" as well as the other two +strings. Another example is given in the discussion of DEFINE above. + +
+All subroutine calls, whether recursive or not, are always treated as atomic +groups. That is, once a subroutine has matched some of the subject string, it +is never re-entered, even if it contains untried alternatives and there is a +subsequent matching failure. Any capturing parentheses that are set during the +subroutine call revert to their previous values afterwards. +
++Processing options such as case-independence are fixed when a subpattern is +defined, so if it is used as a subroutine, such options cannot be changed for +different calls. For example, consider this pattern: +
+ (abc)(?i:(?-1)) ++It matches "abcabc". It does not match "abcABC" because the change of +processing option does not affect the called subpattern. + +
+For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or +a number enclosed either in angle brackets or single quotes, is an alternative +syntax for referencing a subpattern as a subroutine, possibly recursively. Here +are two of the examples used above, rewritten using this syntax: +
+ (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) ) + (sens|respons)e and \g'1'ibility ++PCRE2 supports an extension to Oniguruma: if a number is preceded by a +plus or a minus sign it is taken as a relative reference. For example: +
+ (abc)(?i:\g<-1>) ++Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not +synonymous. The former is a back reference; the latter is a subroutine call. + +
+Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl +code to be obeyed in the middle of matching a regular expression. This makes it +possible, amongst other things, to extract different substrings that match the +same pair of parentheses when there is a repetition. +
++PCRE2 provides a similar feature, but of course it cannot obey arbitrary Perl +code. The feature is called "callout". The caller of PCRE2 provides an external +function by putting its entry point in a match context using the function +pcre2_set_callout() and passing the context to pcre2_match() or +pcre2_dfa_match(). If no match context is passed, or if the callout entry +point is set to NULL, callouts are disabled. +
++Within a regular expression, (?C) indicates the points at which the external +function is to be called. If you want to identify different callout points, you +can put a number less than 256 after the letter C. The default value is zero. +For example, this pattern has two callout points: +
+ (?C1)abc(?C2)def ++If the PCRE2_AUTO_CALLOUT flag is passed to pcre2_compile(), callouts are +automatically installed before each item in the pattern. They are all numbered +255. If there is a conditional group in the pattern whose condition is an +assertion, an additional callout is inserted just before the condition. An +explicit callout may also be set at this position, as in this example: +
+ (?(?C9)(?=a)abc|def) ++Note that this applies only to assertion conditions, not to other types of +condition. + +
+During matching, when PCRE2 reaches a callout point, the external function is +called. It is provided with the number of the callout, the position in the +pattern, and one item of data that is also set in the match block. The callout +function may cause matching to proceed, to backtrack, or to fail. +
++By default, PCRE2 implements a number of optimizations at matching time, and +one side-effect is that sometimes callouts are skipped. If you need all +possible callouts to happen, you need to set options that disable the relevant +optimizations. More details, and a complete description of the interface to the +callout function, are given in the +pcre2callout +documentation. +
++Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which +are still described in the Perl documentation as "experimental and subject to +change or removal in a future version of Perl". It goes on to say: "Their usage +in production code should be noted to avoid problems during upgrades." The same +remarks apply to the PCRE2 features described in this section. +
++The new verbs make use of what was previously invalid syntax: an opening +parenthesis followed by an asterisk. They are generally of the form +(*VERB) or (*VERB:NAME). Some may take either form, possibly behaving +differently depending on whether or not a name is present. A name is any +sequence of characters that does not include a closing parenthesis. The maximum +length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit +libraries. If the name is empty, that is, if the closing parenthesis +immediately follows the colon, the effect is as if the colon were not there. +Any number of these verbs may occur in a pattern. +
++Since these verbs are specifically related to backtracking, most of them can be +used only when the pattern is to be matched using the traditional matching +function, because these use a backtracking algorithm. With the exception of +(*FAIL), which behaves like a failing negative assertion, the backtracking +control verbs cause an error if encountered by the DFA matching function. +
++The behaviour of these verbs in +repeated groups, +assertions, +and in +subpatterns called as subroutines +(whether or not recursively) is documented below. +
++PCRE2 contains some optimizations that are used to speed up matching by running +some checks at the start of each match attempt. For example, it may know the +minimum length of matching subject, or that a particular character must be +present. When one of these optimizations bypasses the running of a match, any +included backtracking verbs will not, of course, be processed. You can suppress +the start-of-match optimizations by setting the PCRE2_NO_START_OPTIMIZE option +when calling pcre2_compile(), or by starting the pattern with +(*NO_START_OPT). There is more discussion of this option in the section +entitled +"Compiling a pattern" +in the +pcre2api +documentation. +
++Experiments with Perl suggest that it too has similar optimizations, sometimes +leading to anomalous results. +
++The following verbs act as soon as they are encountered. They may not be +followed by a name. +
+ (*ACCEPT) ++This verb causes the match to end successfully, skipping the remainder of the +pattern. However, when it is inside a subpattern that is called as a +subroutine, only that subpattern is ended successfully. Matching then continues +at the outer level. If (*ACCEPT) in triggered in a positive assertion, the +assertion succeeds; in a negative assertion, the assertion fails. + +
+If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For +example: +
+ A((?:A|B(*ACCEPT)|C)D) ++This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by +the outer parentheses. +
+ (*FAIL) or (*F) ++This verb causes a matching failure, forcing backtracking to occur. It is +equivalent to (?!) but easier to read. The Perl documentation notes that it is +probably useful only when combined with (?{}) or (??{}). Those are, of course, +Perl features that are not present in PCRE2. The nearest equivalent is the +callout feature, as for example in this pattern: +
+ a+(?C)(*FAIL) ++A match with the string "aaaa" always fails, but the callout is taken before +each backtrack happens (in this example, 10 times). + +
+There is one verb whose main purpose is to track how a match was arrived at, +though it also has a secondary use in conjunction with advancing the match +starting point (see (*SKIP) below). +
+ (*MARK:NAME) or (*:NAME) ++A name is always required with this verb. There may be as many instances of +(*MARK) as you like in a pattern, and their names do not have to be unique. + +
+When a match succeeds, the name of the last-encountered (*MARK:NAME), +(*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the +caller as described in the section entitled +"Other information about the match" +in the +pcre2api +documentation. Here is an example of pcre2test output, where the "mark" +modifier requests the retrieval and outputting of (*MARK) data: +
+ re> /X(*MARK:A)Y|X(*MARK:B)Z/mark + data> XY + 0: XY + MK: A + XZ + 0: XZ + MK: B ++The (*MARK) name is tagged with "MK:" in this output, and in this example it +indicates which of the two alternatives matched. This is a more efficient way +of obtaining this information than putting each alternative in its own +capturing parentheses. + +
+If a verb with a name is encountered in a positive assertion that is true, the +name is recorded and passed back if it is the last-encountered. This does not +happen for negative assertions or failing positive assertions. +
++After a partial match or a failed match, the last encountered name in the +entire match process is returned. For example: +
+ re> /X(*MARK:A)Y|X(*MARK:B)Z/mark + data> XP + No match, mark = B ++Note that in this unanchored example the mark is retained from the match +attempt that started at the letter "X" in the subject. Subsequent match +attempts starting at "P" and then with an empty string do not get as far as the +(*MARK) item, but nevertheless do not reset it. + +
+If you are interested in (*MARK) values after failed matches, you should +probably set the PCRE2_NO_START_OPTIMIZE option +(see above) +to ensure that the match is always attempted. +
++The following verbs do nothing when they are encountered. Matching continues +with what follows, but if there is no subsequent match, causing a backtrack to +the verb, a failure is forced. That is, backtracking cannot pass to the left of +the verb. However, when one of these verbs appears inside an atomic group +(which includes any group that is called as a subroutine) or in an assertion +that is true, its effect is confined to that group, because once the group has +been matched, there is never any backtracking into it. In this situation, +backtracking has to jump to the left of the entire atomic group or assertion. +
++These verbs differ in exactly what kind of failure occurs when backtracking +reaches them. The behaviour described below is what happens when the verb is +not in a subroutine or an assertion. Subsequent sections cover these special +cases. +
+ (*COMMIT) ++This verb, which may not be followed by a name, causes the whole match to fail +outright if there is a later matching failure that causes backtracking to reach +it. Even if the pattern is unanchored, no further attempts to find a match by +advancing the starting point take place. If (*COMMIT) is the only backtracking +verb that is encountered, once it has been passed pcre2_match() is +committed to finding a match at the current starting point, or not at all. For +example: +
+ a+(*COMMIT)b ++This matches "xxaab" but not "aacaab". It can be thought of as a kind of +dynamic anchor, or "I've started, so I must finish." The name of the most +recently passed (*MARK) in the path is passed back when (*COMMIT) forces a +match failure. + +
+If there is more than one backtracking verb in a pattern, a different one that +follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a +match does not always guarantee that a match must be at this starting point. +
++Note that (*COMMIT) at the start of a pattern is not the same as an anchor, +unless PCRE2's start-of-match optimizations are turned off, as shown in this +output from pcre2test: +
+ re> /(*COMMIT)abc/ + data> xyzabc + 0: abc + data> + re> /(*COMMIT)abc/no_start_optimize + data> xyzabc + No match ++For the first pattern, PCRE2 knows that any match must start with "a", so the +optimization skips along the subject to "a" before applying the pattern to the +first set of data. The match attempt then succeeds. The second pattern disables +the optimization that skips along to the first character. The pattern is now +applied starting at "x", and so the (*COMMIT) causes the match to fail without +trying any other starting points. +
+ (*PRUNE) or (*PRUNE:NAME) ++This verb causes the match to fail at the current starting position in the +subject if there is a later matching failure that causes backtracking to reach +it. If the pattern is unanchored, the normal "bumpalong" advance to the next +starting character then happens. Backtracking can occur as usual to the left of +(*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but +if there is no match to the right, backtracking cannot cross (*PRUNE). In +simple cases, the use of (*PRUNE) is just an alternative to an atomic group or +possessive quantifier, but there are some uses of (*PRUNE) that cannot be +expressed in any other way. In an anchored pattern (*PRUNE) has the same effect +as (*COMMIT). + +
+The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE). +It is like (*MARK:NAME) in that the name is remembered for passing back to the +caller. However, (*SKIP:NAME) searches only for names set with (*MARK), +ignoring those set by (*PRUNE) or (*THEN). +
+ (*SKIP) ++This verb, when given without a name, is like (*PRUNE), except that if the +pattern is unanchored, the "bumpalong" advance is not to the next character, +but to the position in the subject where (*SKIP) was encountered. (*SKIP) +signifies that whatever text was matched leading up to it cannot be part of a +successful match. Consider: +
+ a+(*SKIP)b ++If the subject is "aaaac...", after the first match attempt fails (starting at +the first character in the string), the starting point skips on to start the +next attempt at "c". Note that a possessive quantifer does not have the same +effect as this example; although it would suppress backtracking during the +first match attempt, the second attempt would start at the second character +instead of skipping on to "c". +
+ (*SKIP:NAME) ++When (*SKIP) has an associated name, its behaviour is modified. When it is +triggered, the previous path through the pattern is searched for the most +recent (*MARK) that has the same name. If one is found, the "bumpalong" advance +is to the subject position that corresponds to that (*MARK) instead of to where +(*SKIP) was encountered. If no (*MARK) with a matching name is found, the +(*SKIP) is ignored. + +
+Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores +names that are set by (*PRUNE:NAME) or (*THEN:NAME). +
+ (*THEN) or (*THEN:NAME) ++This verb causes a skip to the next innermost alternative when backtracking +reaches it. That is, it cancels any further backtracking within the current +alternative. Its name comes from the observation that it can be used for a +pattern-based if-then-else block: +
+ ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... ++If the COND1 pattern matches, FOO is tried (and possibly further items after +the end of the group if FOO succeeds); on failure, the matcher skips to the +second alternative and tries COND2, without backtracking into COND1. If that +succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no +more alternatives, so there is a backtrack to whatever came before the entire +group. If (*THEN) is not inside an alternation, it acts like (*PRUNE). + +
+The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN). +It is like (*MARK:NAME) in that the name is remembered for passing back to the +caller. However, (*SKIP:NAME) searches only for names set with (*MARK), +ignoring those set by (*PRUNE) and (*THEN). +
++A subpattern that does not contain a | character is just a part of the +enclosing alternative; it is not a nested alternation with only one +alternative. The effect of (*THEN) extends beyond such a subpattern to the +enclosing alternative. Consider this pattern, where A, B, etc. are complex +pattern fragments that do not contain any | characters at this level: +
+ A (B(*THEN)C) | D ++If A and B are matched, but there is a failure in C, matching does not +backtrack into A; instead it moves to the next alternative, that is, D. +However, if the subpattern containing (*THEN) is given an alternative, it +behaves differently: +
+ A (B(*THEN)C | (*FAIL)) | D ++The effect of (*THEN) is now confined to the inner subpattern. After a failure +in C, matching moves to (*FAIL), which causes the whole subpattern to fail +because there are no more alternatives to try. In this case, matching does now +backtrack into A. + +
+Note that a conditional subpattern is not considered as having two +alternatives, because only one is ever used. In other words, the | character in +a conditional subpattern has a different meaning. Ignoring white space, +consider: +
+ ^.*? (?(?=a) a | b(*THEN)c ) ++If the subject is "ba", this pattern does not match. Because .*? is ungreedy, +it initially matches zero characters. The condition (?=a) then fails, the +character "b" is matched, but "c" is not. At this point, matching does not +backtrack to .*? as might perhaps be expected from the presence of the | +character. The conditional subpattern is part of the single alternative that +comprises the whole pattern, and so the match fails. (If there was a backtrack +into .*?, allowing it to match "b", the match would succeed.) + +
+The verbs just described provide four different "strengths" of control when +subsequent matching fails. (*THEN) is the weakest, carrying on the match at the +next alternative. (*PRUNE) comes next, failing the match at the current +starting position, but allowing an advance to the next character (for an +unanchored pattern). (*SKIP) is similar, except that the advance may be more +than one character. (*COMMIT) is the strongest, causing the entire match to +fail. +
++If more than one backtracking verb is present in a pattern, the one that is +backtracked onto first acts. For example, consider this pattern, where A, B, +etc. are complex pattern fragments: +
+ (A(*COMMIT)B(*THEN)C|ABD) ++If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to +fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes +the next alternative (ABD) to be tried. This behaviour is consistent, but is +not always the same as Perl's. It means that if two or more backtracking verbs +appear in succession, all the the last of them has no effect. Consider this +example: +
+ ...(*COMMIT)(*PRUNE)... ++If there is a matching failure to the right, backtracking onto (*PRUNE) causes +it to be triggered, and its action is taken. There can never be a backtrack +onto (*COMMIT). + +
+PCRE2 differs from Perl in its handling of backtracking verbs in repeated +groups. For example, consider: +
+ /(a(*COMMIT)b)+ac/ ++If the subject is "abac", Perl matches, but PCRE2 fails because the (*COMMIT) +in the second repeat of the group acts. + +
+(*FAIL) in an assertion has its normal effect: it forces an immediate +backtrack. +
++(*ACCEPT) in a positive assertion causes the assertion to succeed without any +further processing. In a negative assertion, (*ACCEPT) causes the assertion to +fail without any further processing. +
++The other backtracking verbs are not treated specially if they appear in a +positive assertion. In particular, (*THEN) skips to the next alternative in the +innermost enclosing group that has alternations, whether or not this is within +the assertion. +
++Negative assertions are, however, different, in order to ensure that changing a +positive assertion into a negative assertion changes its result. Backtracking +into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true, +without considering any further alternative branches in the assertion. +Backtracking into (*THEN) causes it to skip to the next enclosing alternative +within the assertion (the normal behaviour), but if the assertion does not have +such an alternative, (*THEN) behaves like (*PRUNE). +
++These behaviours occur whether or not the subpattern is called recursively. +Perl's treatment of subroutines is different in some cases. +
++(*FAIL) in a subpattern called as a subroutine has its normal effect: it forces +an immediate backtrack. +
++(*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to +succeed without any further processing. Matching then continues after the +subroutine call. +
++(*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause +the subroutine match to fail. +
++(*THEN) skips to the next alternative in the innermost enclosing group within +the subpattern that has alternatives. If there is no such group within the +subpattern, (*THEN) causes the subroutine match to fail. +
++pcre2api(3), pcre2callout(3), pcre2matching(3), +pcre2syntax(3), pcre2(3), pcre216(3), pcre232(3). +
+
+Philip Hazel
+
+University Computing Service
+
+Cambridge CB2 3QH, England.
+
+
+Last updated: 19 October 2014
+
+Copyright © 1997-2014 University of Cambridge.
+
+
+Return to the PCRE2 index page. +
diff --git a/doc/html/pcre2test.html b/doc/html/pcre2test.html index 617c582..592d9e0 100644 --- a/doc/html/pcre2test.html +++ b/doc/html/pcre2test.html @@ -148,9 +148,9 @@ to the same value:ebcdic compiled for an EBCDIC environment jit just-in-time support is available - pcre16 the 16-bit library was built - pcre32 the 32-bit library was built - pcre8 the 8-bit library was built + pcre2-16 the 16-bit library was built + pcre2-32 the 32-bit library was built + pcre2-8 the 8-bit library was built unicode Unicode support is availableIf an unknown option is given, an error message is output; the exit code is 0. diff --git a/doc/pcre2.txt b/doc/pcre2.txt index 18e5cc1..2301c8a 100644 --- a/doc/pcre2.txt +++ b/doc/pcre2.txt @@ -8,6 +8,158 @@ pcre2test commands. ----------------------------------------------------------------------------- +PCRE2(3) Library Functions Manual PCRE2(3) + + + +NAME + PCRE2 - Perl-compatible regular expressions (revised API) + +INTRODUCTION + + PCRE2 is the name used for a revised API for the PCRE library, which is + a set of functions, written in C, that implement regular expression + pattern matching using the same syntax and semantics as Perl, with just + a few differences. Some features that appeared in Python and the origi- + nal PCRE before they appeared in Perl are also available using the + Python syntax, there is some support for one or two .NET and Oniguruma + syntax items, and there are options for requesting some minor changes + that give better ECMAScript (aka JavaScript) compatibility. + + The source code for PCRE2 can be compiled to support 8-bit, 16-bit, or + 32-bit code units, which means that up to three separate libraries may + be installed. The original work to extend PCRE to 16-bit and 32-bit + code units was done by Zoltan Herczeg and Christian Persch, respec- + tively. In all three cases, strings can be interpreted either as one + character per code unit, or as UTF-encoded Unicode, with support for + Unicode general category properties. Unicode is optional at build time, + and must be enabled explicitly at run time. The version of Unicode in + use can be discovered by running + + pcre2test -C + + The three libraries contain identical sets of functions, with names + ending in _8, _16, or _32, respectively (for example, pcre2_com- + pile_8()). However, by defining PCRE2_CODE_UNIT_WIDTH to be 8, 16, or + 32, a program that uses just one code unit width can be written using + generic names such as pcre2_compile(), and the documentation is written + assuming that this is the case. + + In addition to the Perl-compatible matching function, PCRE2 contains an + alternative function that matches the same compiled patterns in a dif- + ferent way. In certain circumstances, the alternative function has some + advantages. For a discussion of the two matching algorithms, see the + pcre2matching page. + + Details of exactly which Perl regular expression features are and are + not supported by PCRE2 are given in separate documents. See the + pcre2pattern and pcre2compat pages. There is a syntax summary in the + pcre2syntax page. + + Some features of PCRE2 can be included, excluded, or changed when the + library is built. The pcre2_config() function makes it possible for a + client to discover which features are available. The features them- + selves are described in the pcre2build page. Documentation about build- + ing PCRE2 for various operating systems can be found in the README and + NON-AUTOTOOLS_BUILD files in the source distribution. + + The libraries contains a number of undocumented internal functions and + data tables that are used by more than one of the exported external + functions, but which are not intended for use by external callers. + Their names all begin with "_pcre2", which hopefully will not provoke + any name clashes. In some environments, it is possible to control which + external symbols are exported when a shared library is built, and in + these cases the undocumented symbols are not exported. + + +SECURITY CONSIDERATIONS + + If you are using PCRE2 in a non-UTF application that permits users to + supply arbitrary patterns for compilation, you should be aware of a + feature that allows users to turn on UTF support from within a pattern, + provided that PCRE2 was built with Unicode support. For example, an + 8-bit pattern that begins with "(*UTF)" turns on UTF-8 mode, which + interprets patterns and subjects as strings of UTF-8 code units instead + of individual 8-bit characters. This causes both the pattern and any + data against which it is matched to be checked for UTF-8 validity. If + the data string is very long, such a check might use sufficiently many + resources as to cause your application to lose performance. + + One way of guarding against this possibility is to use the pcre2_pat- + tern_info() function to check the compiled pattern's options for UTF. + Alternatively, you can set the PCRE2_NEVER_UTF option at compile time. + This causes an compile time error if a pattern contains a UTF-setting + sequence. + + If your application is one that supports UTF, be aware that validity + checking can take time. If the same data string is to be matched many + times, you can use the PCRE2_NO_UTF_CHECK option for the second and + subsequent matches to avoid running redundant checks. + + Another way that performance can be hit is by running a pattern that + has a very large search tree against a string that will never match. + Nested unlimited repeats in a pattern are a common example. PCRE2 pro- + vides some protection against this: see the pcre2_set_match_limit() + function in the pcre2api page. + + +USER DOCUMENTATION + + The user documentation for PCRE2 comprises a number of different sec- + tions. In the "man" format, each of these is a separate "man page". In + the HTML format, each is a separate page, linked from the index page. + In the plain text format, the descriptions of the pcre2grep and + pcre2test programs are in files called pcre2grep.txt and pcre2test.txt, + respectively. The remaining sections, except for the pcre2demo section + (which is a program listing), and the short pages for individual func- + tions, are concatenated in pcre2.txt, for ease of searching. The sec- + tions are as follows: + + pcre2 this document FIXME CHECK THIS LIST + pcre2-config show PCRE2 installation configuration information + pcre2api details of PCRE2's native C API + pcre2build building PCRE2 + pcre2callout details of the callout feature + pcre2compat discussion of Perl compatibility + pcre2demo a demonstration C program that uses PCRE2 + pcre2grep description of the pcre2grep command (8-bit only) + pcre2jit discussion of the just-in-time optimization sup- + port + pcre2limits details of size and other limits + pcre2matching discussion of the two matching algorithms + pcre2partial details of the partial matching facility + pcre2pattern syntax and semantics of supported + regular expressions + pcre2perform discussion of performance issues + pcre2posix the POSIX-compatible C API for the 8-bit library + pcre2sample discussion of the pcre2demo program + pcre2stack discussion of stack usage + pcre2syntax quick syntax reference + pcre2test description of the pcre2test testing command + pcre2unicode discussion of Unicode and UTF support + + In the "man" and HTML formats, there is also a short page for each C + library function, listing its arguments and results. + + +AUTHOR + + Philip Hazel + University Computing Service + Cambridge CB2 3QH, England. + + Putting an actual email address here is a spam magnet. If you want to + email me, use my two initials, followed by the two digits 10, at the + domain cam.ac.uk. + + +REVISION + + Last updated: 28 September 2014 + Copyright (c) 1997-2014 University of Cambridge. +------------------------------------------------------------------------------ + + PCRE2API(3) Library Functions Manual PCRE2API(3) @@ -190,7 +342,7 @@ PCRE2 NATIVE API AUXILIARY FUNCTIONS int pcre2_pattern_info(const pcre2 *code, uint32_t what, void *where); - int pcre2_config(uint32_t what, void *where, PCRE2_SIZE length); + int pcre2_config(uint32_t what, void *where); PCRE2 8-BIT, 16-BIT, AND 32-BIT LIBRARIES @@ -670,7 +822,7 @@ PCRE2 CONTEXTS CHECKING BUILD-TIME OPTIONS - int pcre2_config(uint32_t what, void *where, PCRE2_SIZE length); + int pcre2_config(uint32_t what, void *where); The function pcre2_config() makes it possible for a PCRE2 client to discover which optional features have been compiled into the PCRE2 @@ -679,20 +831,23 @@ CHECKING BUILD-TIME OPTIONS The first argument for pcre2_config() specifies which information is required. The second argument is a pointer to memory into which the - information is placed, with the final argument giving the length of - this memory in bytes. For calls that return numerical values, where - should point to appropriately aligned memory, with length set to at - least the "sizeof" the data type. + information is placed. If NULL is passed, the function returns the + amount of memory that is needed for the requested information. For + calls that return numerical values, the value is in bytes; when + requesting these values, where should point to appropriately aligned + memory. For calls that return strings, the required length is given in + code units, not counting the terminating zero. - The returned value from pcre2_config() is zero on success, or the nega- - tive error code PCRE2_ERROR_BADOPTION if the value in the first argu- - ment is not recognized. The following information is available: + When requesting information, the returned value from pcre2_config() is + non-negative on success, or the negative error code PCRE2_ERROR_BADOP- + TION if the value in the first argument is not recognized. The follow- + ing information is available: PCRE2_CONFIG_BSR The output is an integer whose value indicates what character sequences - the \R escape sequence matches by default. A value of 0 means that \R - matches any Unicode line ending sequence; a value of 1 means that \R + the \R escape sequence matches by default. A value of 0 means that \R + matches any Unicode line ending sequence; a value of 1 means that \R matches only CR, LF, or CRLF. The default can be overridden when a pat- tern is compiled or matched. @@ -703,10 +858,13 @@ CHECKING BUILD-TIME OPTIONS PCRE2_CONFIG_JITTARGET - FIXME: this needs sorting out once JIT is implemented. If JIT support - is available, the string contains the name of the architecture for - which the JIT compiler is configured, for example "x86 32bit (little - endian + unaligned)". If JIT support is not available, FIXME. + The where argument should point to a buffer that is at least 48 code + units long. (The exact length needed can be found by calling pcre2_con- + fig() with where set to NULL.) The buffer is filled with a string that + contains the name of the architecture for which the JIT compiler is + configured, for example "x86 32bit (little endian + unaligned)". If JIT + support is not available, PCRE2_ERROR_BADOPTION is returned, otherwise + the length of the string, in code units, is returned. PCRE2_CONFIG_LINKSIZE @@ -772,10 +930,12 @@ CHECKING BUILD-TIME OPTIONS PCRE2_CONFIG_UNICODE_VERSION The where argument should point to a buffer that is at least 24 code - units long. If PCRE2 has been compiled without Unicode support, this is - filled with the text "Unicode not supported". Otherwise, the Unicode - version string (for example, "7.0.0") is returnd. The string is zero- - terminated. + units long. (The exact length needed can be found by calling pcre2_con- + fig() with where set to NULL.) If PCRE2 has been compiled without Uni- + code support, the buffer is filled with the text "Unicode not sup- + ported". Otherwise, the Unicode version string (for example, "7.0.0") + is inserted. The string is zero-terminated. The function returns the + length of the string in code units. PCRE2_CONFIG_UNICODE @@ -786,8 +946,10 @@ CHECKING BUILD-TIME OPTIONS PCRE2_CONFIG_VERSION The where argument should point to a buffer that is at least 12 code - units long. It is filled with the PCRE2 version string, zero-termi- - nated. + units long. (The exact length needed can be found by calling pcre2_con- + fig() with where set to NULL.) The buffer is filled with the PCRE2 ver- + sion string, zero-terminated. The length of the string (in code units) + is returned. COMPILING A PATTERN @@ -1227,18 +1389,19 @@ INFORMATION ABOUT A COMPILED PATTERN The pcre2_pattern_info() function returns information about a compiled pattern. The first argument is a pointer to the compiled pattern. The second argument specifies which piece of information is required, and - the third argument is a pointer to a variable to receive the data. The - yield of the function is zero for success, or one of the following neg- - ative numbers: + the third argument is a pointer to a variable to receive the data. If + the third argument is NULL, the first argument is ignored, and the + function returns the size in bytes of the variable that is required for + the information requested. Otherwise, The yield of the function is + zero for success, or one of the following negative numbers: PCRE2_ERROR_NULL the argument code was NULL - the argument where was NULL PCRE2_ERROR_BADMAGIC the "magic number" was not found PCRE2_ERROR_BADOPTION the value of what was invalid PCRE2_ERROR_UNSET the requested field is not set The "magic number" is placed at the start of each compiled pattern as - an simple check against passing an arbitrary memory pointer. Here is a + an simple check against passing an arbitrary memory pointer. Here is a typical call of pcre2_pattern_info(), to obtain the length of the com- piled pattern: @@ -2363,7 +2526,459 @@ AUTHOR REVISION - Last updated: 14 October 2014 + Last updated: 16 October 2014 + Copyright (c) 1997-2014 University of Cambridge. +------------------------------------------------------------------------------ + + +PCRE2BUILD(3) Library Functions Manual PCRE2BUILD(3) + + + +NAME + PCRE2 - Perl-compatible regular expressions (revised API) + +BUILDING PCRE2 + + PCRE2 is distributed with a configure script that can be used to build + the library in Unix-like environments using the applications known as + Autotools. Also in the distribution are files to support building using + CMake instead of configure. The text file README contains general + information about building with Autotools (some of which is repeated + below), and also has some comments about building on various operating + systems. There is a lot more information about building PCRE2 without + using Autotools (including information about using CMake and building + "by hand") in the text file called NON-AUTOTOOLS-BUILD. You should + consult this file as well as the README file if you are building in a + non-Unix-like environment. + + +PCRE2 BUILD-TIME OPTIONS + + The rest of this document describes the optional features of PCRE2 that + can be selected when the library is compiled. It assumes use of the + configure script, where the optional features are selected or dese- + lected by providing options to configure before running the make com- + mand. However, the same options can be selected in both Unix-like and + non-Unix-like environments if you are using CMake instead of configure + to build PCRE2. + + If you are not using Autotools or CMake, option selection can be done + by editing the config.h file, or by passing parameter settings to the + compiler, as described in NON-AUTOTOOLS-BUILD. + + The complete list of options for configure (which includes the standard + ones such as the selection of the installation directory) can be + obtained by running + + ./configure --help + + The following sections include descriptions of options whose names + begin with --enable or --disable. These settings specify changes to the + defaults for the configure command. Because of the way that configure + works, --enable and --disable always come in pairs, so the complemen- + tary option always exists as well, but as it specifies the default, it + is not described. + + +BUILDING 8-BIT, 16-BIT AND 32-BIT LIBRARIES + + By default, a library called libpcre2-8 is built, containing functions + that take string arguments contained in vectors of bytes, interpreted + either as single-byte characters, or UTF-8 strings. You can also build + two other libraries, called libpcre2-16 and libpcre2-32, which process + strings that are contained in vectors of 16-bit and 32-bit code units, + respectively. These can be interpreted either as single-unit characters + or UTF-16/UTF-32 strings. To build these additional libraries, add one + or both of the following to the configure command: + + --enable-pcre16 + --enable-pcre32 + + If you do not want the 8-bit library, add + + --disable-pcre8 + + as well. At least one of the three libraries must be built. Note that + the POSIX wrapper is for the 8-bit library only, and that pcre2grep is + an 8-bit program. Neither of these are built if you select only the + 16-bit or 32-bit libraries. + + +BUILDING SHARED AND STATIC LIBRARIES + + The Autotools PCRE2 building process uses libtool to build both shared + and static libraries by default. You can suppress one of these by + adding one of + + --disable-shared + --disable-static + + to the configure command, as required. + + +Unicode and UTF SUPPORT + + To build PCRE2 with support for Unicode and UTF character strings, add + + --enable-unicode + + to the configure command. This setting applies to all three libraries, + adding support for UTF-8 to the 8-bit library, support for UTF-16 to + the 16-bit library, and support for UTF-32 to the to the 32-bit + library. It is not possible to build one library with UTF support and + another without in the same configuration. + + Of itself, this setting does not make PCRE2 treat strings as UTF-8, + UTF-16 or UTF-32. As well as compiling PCRE2 with this option, you also + have have to set the PCRE2_UTF option when you call pcre2_compile() to + compile a pattern. + + If you set --enable-unicode when compiling in an EBCDIC environment, + PCRE2 expects its input to be either ASCII or UTF-8 (depending on the + run-time option). It is not possible to support both EBCDIC and UTF-8 + codes in the same version of the library. Consequently, --enable-uni- + code and --enable-ebcdic are mutually exclusive. + + UTF support allows the libraries to process character codepoints up to + 0x10ffff in the strings that they handle. It also provides support for + accessing the properties of such characters, using pattern escapes such + as \P, \p, and \X. Only the general category properties such as Lu and + Nd are supported. Details are given in the pcre2pattern documentation. + + +JUST-IN-TIME COMPILER SUPPORT + + Just-in-time compiler support is included in the build by specifying + + --enable-jit + + This support is available only for certain hardware architectures. If + this option is set for an unsupported architecture, a compile time + error occurs. See the pcre2jit documentation for a discussion of JIT + usage. When JIT support is enabled, pcre2grep automatically makes use + of it, unless you add + + --disable-pcre2grep-jit + + to the "configure" command. + + +CODE VALUE OF NEWLINE + + By default, PCRE2 interprets the linefeed (LF) character as indicating + the end of a line. This is the normal newline character on Unix-like + systems. You can compile PCRE2 to use carriage return (CR) instead, by + adding + + --enable-newline-is-cr + + to the configure command. There is also a --enable-newline-is-lf + option, which explicitly specifies linefeed as the newline character. + + Alternatively, you can specify that line endings are to be indicated by + the two character sequence CRLF. If you want this, add + + --enable-newline-is-crlf + + to the configure command. There is a fourth option, specified by + + --enable-newline-is-anycrlf + + which causes PCRE2 to recognize any of the three sequences CR, LF, or + CRLF as indicating a line ending. Finally, a fifth option, specified by + + --enable-newline-is-any + + causes PCRE2 to recognize any Unicode newline sequence. + + Whatever line ending convention is selected when PCRE2 is built can be + overridden when the library functions are called. At build time it is + conventional to use the standard for your operating system. + + +WHAT \R MATCHES + + By default, the sequence \R in a pattern matches any Unicode newline + sequence, whatever has been selected as the line ending sequence. If + you specify + + --enable-bsr-anycrlf + + the default is changed so that \R matches only CR, LF, or CRLF. What- + ever is selected when PCRE2 is built can be overridden when the library + functions are called. + + +HANDLING VERY LARGE PATTERNS + + Within a compiled pattern, offset values are used to point from one + part to another (for example, from an opening parenthesis to an alter- + nation metacharacter). By default, in the 8-bit and 16-bit libraries, + two-byte values are used for these offsets, leading to a maximum size + for a compiled pattern of around 64K. This is sufficient to handle all + but the most gigantic patterns. Nevertheless, some people do want to + process truly enormous patterns, so it is possible to compile PCRE2 to + use three-byte or four-byte offsets by adding a setting such as + + --with-link-size=3 + + to the configure command. The value given must be 2, 3, or 4. For the + 16-bit library, a value of 3 is rounded up to 4. In these libraries, + using longer offsets slows down the operation of PCRE2 because it has + to load additional data when handling them. For the 32-bit library the + value is always 4 and cannot be overridden; the value of --with-link- + size is ignored. + + +AVOIDING EXCESSIVE STACK USAGE + + When matching with the pcre2_match() function, PCRE2 implements back- + tracking by making recursive calls to an internal function called + match(). In environments where the size of the stack is limited, this + can severely limit PCRE2's operation. (The Unix environment does not + usually suffer from this problem, but it may sometimes be necessary to + increase the maximum stack size. There is a discussion in the + pcre2stack documentation.) An alternative approach to recursion that + uses memory from the heap to remember data, instead of using recursive + function calls, has been implemented to work round the problem of lim- + ited stack size. If you want to build a version of PCRE2 that works + this way, add + + --disable-stack-for-recursion + + to the configure command. By default, the system functions malloc() and + free() are called to manage the heap memory that is required, but cus- + tom memory management functions can be called instead. PCRE2 runs + noticeably more slowly when built in this way. This option affects only + the pcre2_match() function; it is not relevant for pcre2_dfa_match(). + + +LIMITING PCRE2 RESOURCE USAGE + + Internally, PCRE2 has a function called match(), which it calls repeat- + edly (sometimes recursively) when matching a pattern with the + pcre2_match() function. By controlling the maximum number of times this + function may be called during a single matching operation, a limit can + be placed on the resources used by a single call to pcre2_match(). The + limit can be changed at run time, as described in the pcre2api documen- + tation. The default is 10 million, but this can be changed by adding a + setting such as + + --with-match-limit=500000 + + to the configure command. This setting has no effect on the + pcre2_dfa_match() matching function. + + In some environments it is desirable to limit the depth of recursive + calls of match() more strictly than the total number of calls, in order + to restrict the maximum amount of stack (or heap, if --disable-stack- + for-recursion is specified) that is used. A second limit controls this; + it defaults to the value that is set for --with-match-limit, which + imposes no additional constraints. However, you can set a lower limit + by adding, for example, + + --with-match-limit-recursion=10000 + + to the configure command. This value can also be overridden at run + time. + + +CREATING CHARACTER TABLES AT BUILD TIME + + PCRE2 uses fixed tables for processing characters whose code points are + less than 256. By default, PCRE2 is built with a set of tables that are + distributed in the file src/pcre2_chartables.c.dist. These tables are + for ASCII codes only. If you add + + --enable-rebuild-chartables + + to the configure command, the distributed tables are no longer used. + Instead, a program called dftables is compiled and run. This outputs + the source for new set of tables, created in the default locale of your + C run-time system. (This method of replacing the tables does not work + if you are cross compiling, because dftables is run on the local host. + If you need to create alternative tables when cross compiling, you will + have to do so "by hand".) + + +USING EBCDIC CODE + + PCRE2 assumes by default that it will run in an environment where the + character code is ASCII (or Unicode, which is a superset of ASCII). + This is the case for most computer operating systems. PCRE2 can, how- + ever, be compiled to run in an EBCDIC environment by adding + + --enable-ebcdic + + to the configure command. This setting implies --enable-rebuild-charta- + bles. You should only use it if you know that you are in an EBCDIC + environment (for example, an IBM mainframe operating system). The + --enable-ebcdic option is incompatible with --enable-unicode. + + The EBCDIC character that corresponds to an ASCII LF is assumed to have + the value 0x15 by default. However, in some EBCDIC environments, 0x25 + is used. In such an environment you should use + + --enable-ebcdic-nl25 + + as well as, or instead of, --enable-ebcdic. The EBCDIC character for CR + has the same value as in ASCII, namely, 0x0d. Whichever of 0x15 and + 0x25 is not chosen as LF is made to correspond to the Unicode NEL char- + acter (which, in Unicode, is 0x85). + + The options that select newline behaviour, such as --enable-newline-is- + cr, and equivalent run-time options, refer to these character values in + an EBCDIC environment. + + +PCRE2GREP OPTIONS FOR COMPRESSED FILE SUPPORT + + By default, pcre2grep reads all files as plain text. You can build it + so that it recognizes files whose names end in .gz or .bz2, and reads + them with libz or libbz2, respectively, by adding one or both of + + --enable-pcre2grep-libz + --enable-pcre2grep-libbz2 + + to the configure command. These options naturally require that the rel- + evant libraries are installed on your system. Configuration will fail + if they are not. + + +PCRE2GREP BUFFER SIZE + + pcre2grep uses an internal buffer to hold a "window" on the file it is + scanning, in order to be able to output "before" and "after" lines when + it finds a match. The size of the buffer is controlled by a parameter + whose default value is 20K. The buffer itself is three times this size, + but because of the way it is used for holding "before" lines, the long- + est line that is guaranteed to be processable is the parameter size. + You can change the default parameter value by adding, for example, + + --with-pcre2grep-bufsize=50K + + to the configure command. The caller of pcre2grep can, however, over- + ride this value by specifying a run-time option. + + +PCRE2TEST OPTION FOR LIBREADLINE SUPPORT + + If you add one of + + --enable-pcre2test-libreadline + --enable-pcre2test-libedit + + to the configure command, pcre2test is linked with the libreadline + orlibedit library, respectively, and when its input is from a terminal, + it reads it using the readline() function. This provides line-editing + and history facilities. Note that libreadline is GPL-licensed, so if + you distribute a binary of pcre2test linked in this way, there may be + licensing issues. These can be avoided by linking with libedit (which + has a BSD licence) instead. + + Setting this option causes the -lreadline option to be added to the + pcre2test build. In many operating environments with a sytem-installed + readline library this is sufficient. However, in some environments + (e.g. if an unmodified distribution version of readline is in use), + some extra configuration may be necessary. The INSTALL file for + libreadline says this: + + "Readline uses the termcap functions, but does not link with + the termcap or curses library itself, allowing applications + which link with readline the to choose an appropriate library." + + If your environment has not been set up so that an appropriate library + is automatically included, you may need to add something like + + LIBS="-ncurses" + + immediately before the configure command. + + +DEBUGGING WITH VALGRIND SUPPORT + + By adding the + + --enable-valgrind + + option to to the configure command, PCRE2 will use valgrind annotations + to mark certain memory regions as unaddressable. This allows it to + detect invalid memory accesses, and is mostly useful for debugging + PCRE2 itself. + + +CODE COVERAGE REPORTING + + If your C compiler is gcc, you can build a version of PCRE2 that can + generate a code coverage report for its test suite. To enable this, you + must install lcov version 1.6 or above. Then specify + + --enable-coverage + + to the configure command and build PCRE2 in the usual way. + + Note that using ccache (a caching C compiler) is incompatible with code + coverage reporting. If you have configured ccache to run automatically + on your system, you must set the environment variable + + CCACHE_DISABLE=1 + + before running make to build PCRE2, so that ccache is not used. + + When --enable-coverage is used, the following addition targets are + added to the Makefile: + + make coverage + + This creates a fresh coverage report for the PCRE2 test suite. It is + equivalent to running "make coverage-reset", "make coverage-baseline", + "make check", and then "make coverage-report". + + make coverage-reset + + This zeroes the coverage counters, but does nothing else. + + make coverage-baseline + + This captures baseline coverage information. + + make coverage-report + + This creates the coverage report. + + make coverage-clean-report + + This removes the generated coverage report without cleaning the cover- + age data itself. + + make coverage-clean-data + + This removes the captured coverage data without removing the coverage + files created at compile time (*.gcno). + + make coverage-clean + + This cleans all coverage data including the generated coverage report. + For more information about code coverage, see the gcov and lcov docu- + mentation. + + +SEE ALSO + + pcre2api(3), pcre2_config(3). + + +AUTHOR + + Philip Hazel + University Computing Service + Cambridge CB2 3QH, England. + + +REVISION + + Last updated: 28 September 2014 Copyright (c) 1997-2014 University of Cambridge. ------------------------------------------------------------------------------ @@ -2604,6 +3219,1274 @@ REVISION ------------------------------------------------------------------------------ +PCRE2COMPAT(3) Library Functions Manual PCRE2COMPAT(3) + + + +NAME + PCRE2 - Perl-compatible regular expressions (revised API) + +DIFFERENCES BETWEEN PCRE2 AND PERL + + This document describes the differences in the ways that PCRE2 and Perl + handle regular expressions. The differences described here are with + respect to Perl versions 5.10 and above. + + 1. PCRE2 has only a subset of Perl's Unicode support. Details of what + it does have are given in the pcre2unicode page. + + 2. PCRE2 allows repeat quantifiers only on parenthesized assertions, + but they do not mean what you might think. For example, (?!a){3} does + not assert that the next three characters are not "a". It just asserts + that the next character is not "a" three times (in principle: PCRE2 + optimizes this to run the assertion just once). Perl allows repeat + quantifiers on other assertions such as \b, but these do not seem to + have any use. + + 3. Capturing subpatterns that occur inside negative lookahead asser- + tions are counted, but their entries in the offsets vector are never + set. Perl sometimes (but not always) sets its numerical variables from + inside negative assertions. + + 4. The following Perl escape sequences are not supported: \l, \u, \L, + \U, and \N when followed by a character name or Unicode value. (\N on + its own, matching a non-newline character, is supported.) In fact these + are implemented by Perl's general string-handling and are not part of + its pattern matching engine. If any of these are encountered by PCRE2, + an error is generated by default. However, if the PCRE2_ALT_BSUX option + is set, \U and \u are interpreted as ECMAScript interprets them. + + 5. The Perl escape sequences \p, \P, and \X are supported only if PCRE2 + is built with Unicode support. The properties that can be tested with + \p and \P are limited to the general category properties such as Lu and + Nd, script names such as Greek or Han, and the derived properties Any + and L&. PCRE2 does support the Cs (surrogate) property, which Perl does + not; the Perl documentation says "Because Perl hides the need for the + user to understand the internal representation of Unicode characters, + there is no need to implement the somewhat messy concept of surro- + gates." + + 6. PCRE2 does support the \Q...\E escape for quoting substrings. Char- + acters in between are treated as literals. This is slightly different + from Perl in that $ and @ are also handled as literals inside the + quotes. In Perl, they cause variable interpolation (but of course PCRE2 + does not have variables). Note the following examples: + + Pattern PCRE2 matches Perl matches + + \Qabc$xyz\E abc$xyz abc followed by the + contents of $xyz + \Qabc\$xyz\E abc\$xyz abc\$xyz + \Qabc\E\$\Qxyz\E abc$xyz abc$xyz + + The \Q...\E sequence is recognized both inside and outside character + classes. + + 7. Fairly obviously, PCRE2 does not support the (?{code}) and + (??{code}) constructions. However, there is support for recursive pat- + terns. This is not available in Perl 5.8, but it is in Perl 5.10. Also, + the PCRE2 "callout" feature allows an external function to be called + during pattern matching. See the pcre2callout documentation for + details. + + 8. Subpatterns that are called as subroutines (whether or not recur- + sively) are always treated as atomic groups in PCRE2. This is like + Python, but unlike Perl. Captured values that are set outside a sub- + routine call can be reference from inside in PCRE2, but not in Perl. + There is a discussion that explains these differences in more detail in + the section on recursion differences from Perl in the pcre2pattern + page. + + 9. If any of the backtracking control verbs are used in a subpattern + that is called as a subroutine (whether or not recursively), their + effect is confined to that subpattern; it does not extend to the sur- + rounding pattern. This is not always the case in Perl. In particular, + if (*THEN) is present in a group that is called as a subroutine, its + action is limited to that group, even if the group does not contain any + | characters. Note that such subpatterns are processed as anchored at + the point where they are tested. + + 10. If a pattern contains more than one backtracking control verb, the + first one that is backtracked onto acts. For example, in the pattern + A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure + in C triggers (*PRUNE). Perl's behaviour is more complex; in many cases + it is the same as PCRE2, but there are examples where it differs. + + 11. Most backtracking verbs in assertions have their normal actions. + They are not confined to the assertion. + + 12. There are some differences that are concerned with the settings of + captured strings when part of a pattern is repeated. For example, + matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2 + unset, but in PCRE2 it is set to "b". + + 13. PCRE2's handling of duplicate subpattern numbers and duplicate sub- + pattern names is not as general as Perl's. This is a consequence of the + fact the PCRE2 works internally just with numbers, using an external + table to translate between numbers and names. In particular, a pattern + such as (?|(?A)|(?