278 lines
10 KiB
Groff
278 lines
10 KiB
Groff
.TH PCRE2UNICODE 3 "20 April 2017" "PCRE2 10.30"
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.SH NAME
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PCRE - Perl-compatible regular expressions (revised API)
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.SH "UNICODE AND UTF SUPPORT"
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.rs
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.sp
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When PCRE2 is built with Unicode support (which is the default), it has
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knowledge of Unicode character properties and can process text strings in
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UTF-8, UTF-16, or UTF-32 format (depending on the code unit width). However, by
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default, PCRE2 assumes that one code unit is one character. To process a
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pattern as a UTF string, where a character may require more than one code unit,
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you must call
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.\" HREF
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\fBpcre2_compile()\fP
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.\"
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with the PCRE2_UTF option flag, or the pattern must start with the sequence
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(*UTF). When either of these is the case, both the pattern and any subject
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strings that are matched against it are treated as UTF strings instead of
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strings of individual one-code-unit characters.
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.P
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If you do not need Unicode support you can build PCRE2 without it, in which
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case the library will be smaller.
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.
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.
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.SH "UNICODE PROPERTY SUPPORT"
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.rs
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.sp
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When PCRE2 is built with Unicode support, the escape sequences \ep{..},
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\eP{..}, and \eX can be used. The Unicode properties that can be tested are
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limited to the general category properties such as Lu for an upper case letter
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or Nd for a decimal number, the Unicode script names such as Arabic or Han, and
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the derived properties Any and L&. Full lists are given in the
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.\" HREF
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\fBpcre2pattern\fP
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.\"
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and
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.\" HREF
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\fBpcre2syntax\fP
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.\"
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documentation. Only the short names for properties are supported. For example,
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\ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.
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Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
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compatibility with Perl 5.6. PCRE2 does not support this.
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.
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.
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.SH "WIDE CHARACTERS AND UTF MODES"
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.rs
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.sp
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Codepoints less than 256 can be specified in patterns by either braced or
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unbraced hexadecimal escape sequences (for example, \ex{b3} or \exb3). Larger
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values have to use braced sequences. Unbraced octal code points up to \e777 are
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also recognized; larger ones can be coded using \eo{...}.
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.P
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In UTF modes, repeat quantifiers apply to complete UTF characters, not to
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individual code units.
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.P
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In UTF modes, the dot metacharacter matches one UTF character instead of a
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single code unit.
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.P
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The escape sequence \eC can be used to match a single code unit in a UTF mode,
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but its use can lead to some strange effects because it breaks up multi-unit
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characters (see the description of \eC in the
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.\" HREF
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\fBpcre2pattern\fP
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.\"
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documentation).
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.P
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The use of \eC is not supported by the alternative matching function
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\fBpcre2_dfa_match()\fP when in UTF-8 or UTF-16 mode, that is, when a character
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may consist of more than one code unit. The use of \eC in these modes provokes
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a match-time error. Also, the JIT optimization does not support \eC in these
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modes. If JIT optimization is requested for a UTF-8 or UTF-16 pattern that
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contains \eC, it will not succeed, and so when \fBpcre2_match()\fP is called,
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the matching will be carried out by the normal interpretive function.
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.P
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The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly test
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characters of any code value, but, by default, the characters that PCRE2
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recognizes as digits, spaces, or word characters remain the same set as in
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non-UTF mode, all with code points less than 256. This remains true even when
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PCRE2 is built to include Unicode support, because to do otherwise would slow
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down matching in many common cases. Note that this also applies to \eb
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and \eB, because they are defined in terms of \ew and \eW. If you want
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to test for a wider sense of, say, "digit", you can use explicit Unicode
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property tests such as \ep{Nd}. Alternatively, if you set the PCRE2_UCP option,
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the way that the character escapes work is changed so that Unicode properties
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are used to determine which characters match. There are more details in the
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section on
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.\" HTML <a href="pcre2pattern.html#genericchartypes">
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.\" </a>
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generic character types
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.\"
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in the
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.\" HREF
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\fBpcre2pattern\fP
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.\"
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documentation.
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.P
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Similarly, characters that match the POSIX named character classes are all
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low-valued characters, unless the PCRE2_UCP option is set.
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.P
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However, the special horizontal and vertical white space matching escapes (\eh,
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\eH, \ev, and \eV) do match all the appropriate Unicode characters, whether or
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not PCRE2_UCP is set.
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.
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.
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.SH "CASE-EQUIVALENCE IN UTF MODES"
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.rs
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.sp
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Case-insensitive matching in a UTF mode makes use of Unicode properties except
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for characters whose code points are less than 128 and that have at most two
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case-equivalent values. For these, a direct table lookup is used for speed. A
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few Unicode characters such as Greek sigma have more than two codepoints that
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are case-equivalent, and these are treated as such.
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.
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.
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.SH "VALIDITY OF UTF STRINGS"
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.rs
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.sp
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When the PCRE2_UTF option is set, the strings passed as patterns and subjects
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are (by default) checked for validity on entry to the relevant functions.
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If an invalid UTF string is passed, an negative error code is returned. The
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code unit offset to the offending character can be extracted from the match
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data block by calling \fBpcre2_get_startchar()\fP, which is used for this
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purpose after a UTF error.
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.P
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UTF-16 and UTF-32 strings can indicate their endianness by special code knows
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as a byte-order mark (BOM). The PCRE2 functions do not handle this, expecting
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strings to be in host byte order.
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.P
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A UTF string is checked before any other processing takes place. In the case of
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\fBpcre2_match()\fP and \fBpcre2_dfa_match()\fP calls with a non-zero starting
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offset, the check is applied only to that part of the subject that could be
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inspected during matching, and there is a check that the starting offset points
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to the first code unit of a character or to the end of the subject. If there
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are no lookbehind assertions in the pattern, the check starts at the starting
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offset. Otherwise, it starts at the length of the longest lookbehind before the
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starting offset, or at the start of the subject if there are not that many
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characters before the starting offset. Note that the sequences \eb and \eB are
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one-character lookbehinds.
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.P
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In addition to checking the format of the string, there is a check to ensure
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that all code points lie in the range U+0 to U+10FFFF, excluding the surrogate
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area. The so-called "non-character" code points are not excluded because
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Unicode corrigendum #9 makes it clear that they should not be.
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.P
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Characters in the "Surrogate Area" of Unicode are reserved for use by UTF-16,
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where they are used in pairs to encode code points with values greater than
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0xFFFF. The code points that are encoded by UTF-16 pairs are available
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independently in the UTF-8 and UTF-32 encodings. (In other words, the whole
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surrogate thing is a fudge for UTF-16 which unfortunately messes up UTF-8 and
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UTF-32.)
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.P
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In some situations, you may already know that your strings are valid, and
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therefore want to skip these checks in order to improve performance, for
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example in the case of a long subject string that is being scanned repeatedly.
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If you set the PCRE2_NO_UTF_CHECK option at compile time or at match time,
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PCRE2 assumes that the pattern or subject it is given (respectively) contains
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only valid UTF code unit sequences.
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.P
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Passing PCRE2_NO_UTF_CHECK to \fBpcre2_compile()\fP just disables the check for
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the pattern; it does not also apply to subject strings. If you want to disable
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the check for a subject string you must pass this option to \fBpcre2_match()\fP
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or \fBpcre2_dfa_match()\fP.
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.P
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If you pass an invalid UTF string when PCRE2_NO_UTF_CHECK is set, the result
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is undefined and your program may crash or loop indefinitely.
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.
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.
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.\" HTML <a name="utf8strings"></a>
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.SS "Errors in UTF-8 strings"
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.rs
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.sp
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The following negative error codes are given for invalid UTF-8 strings:
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.sp
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PCRE2_ERROR_UTF8_ERR1
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PCRE2_ERROR_UTF8_ERR2
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PCRE2_ERROR_UTF8_ERR3
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PCRE2_ERROR_UTF8_ERR4
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PCRE2_ERROR_UTF8_ERR5
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.sp
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The string ends with a truncated UTF-8 character; the code specifies how many
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bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8 characters to be
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no longer than 4 bytes, the encoding scheme (originally defined by RFC 2279)
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allows for up to 6 bytes, and this is checked first; hence the possibility of
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4 or 5 missing bytes.
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.sp
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PCRE2_ERROR_UTF8_ERR6
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PCRE2_ERROR_UTF8_ERR7
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PCRE2_ERROR_UTF8_ERR8
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PCRE2_ERROR_UTF8_ERR9
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PCRE2_ERROR_UTF8_ERR10
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.sp
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The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of the
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character do not have the binary value 0b10 (that is, either the most
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significant bit is 0, or the next bit is 1).
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.sp
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PCRE2_ERROR_UTF8_ERR11
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PCRE2_ERROR_UTF8_ERR12
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.sp
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A character that is valid by the RFC 2279 rules is either 5 or 6 bytes long;
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these code points are excluded by RFC 3629.
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.sp
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PCRE2_ERROR_UTF8_ERR13
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.sp
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A 4-byte character has a value greater than 0x10fff; these code points are
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excluded by RFC 3629.
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.sp
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PCRE2_ERROR_UTF8_ERR14
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.sp
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A 3-byte character has a value in the range 0xd800 to 0xdfff; this range of
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code points are reserved by RFC 3629 for use with UTF-16, and so are excluded
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from UTF-8.
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.sp
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PCRE2_ERROR_UTF8_ERR15
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PCRE2_ERROR_UTF8_ERR16
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PCRE2_ERROR_UTF8_ERR17
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PCRE2_ERROR_UTF8_ERR18
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PCRE2_ERROR_UTF8_ERR19
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.sp
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A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes for a
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value that can be represented by fewer bytes, which is invalid. For example,
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the two bytes 0xc0, 0xae give the value 0x2e, whose correct coding uses just
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one byte.
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.sp
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PCRE2_ERROR_UTF8_ERR20
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.sp
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The two most significant bits of the first byte of a character have the binary
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value 0b10 (that is, the most significant bit is 1 and the second is 0). Such a
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byte can only validly occur as the second or subsequent byte of a multi-byte
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character.
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.sp
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PCRE2_ERROR_UTF8_ERR21
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.sp
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The first byte of a character has the value 0xfe or 0xff. These values can
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never occur in a valid UTF-8 string.
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.
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.
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.\" HTML <a name="utf16strings"></a>
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.SS "Errors in UTF-16 strings"
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.rs
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.sp
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The following negative error codes are given for invalid UTF-16 strings:
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.sp
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PCRE2_ERROR_UTF16_ERR1 Missing low surrogate at end of string
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PCRE2_ERROR_UTF16_ERR2 Invalid low surrogate follows high surrogate
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PCRE2_ERROR_UTF16_ERR3 Isolated low surrogate
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.sp
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.
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.
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.\" HTML <a name="utf32strings"></a>
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.SS "Errors in UTF-32 strings"
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.rs
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.sp
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The following negative error codes are given for invalid UTF-32 strings:
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.sp
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PCRE2_ERROR_UTF32_ERR1 Surrogate character (0xd800 to 0xdfff)
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PCRE2_ERROR_UTF32_ERR2 Code point is greater than 0x10ffff
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.sp
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.
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.
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.SH AUTHOR
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.rs
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.sp
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.nf
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Philip Hazel
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University Computing Service
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Cambridge, England.
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.fi
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.
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.
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.SH REVISION
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.rs
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.sp
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.nf
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Last updated: 20 April 2017
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Copyright (c) 1997-2017 University of Cambridge.
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.fi
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