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Clarify relative refence semantics.

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ph10 2015-11-10 14:50:51 +00:00
parent 961e72941b
commit fb6717e738
1 changed files with 22 additions and 2 deletions
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doc/pcre2pattern.3
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@ -1,4 +1,4 @@
.TH PCRE2PATTERN 3 "01 November 2015" "PCRE2 10.21" .TH PCRE2PATTERN 3 "10 November 2015" "PCRE2 10.21"
.SH NAME .SH NAME
PCRE2 - Perl-compatible regular expressions (revised API) PCRE2 - Perl-compatible regular expressions (revised API)
.SH "PCRE2 REGULAR EXPRESSION DETAILS" .SH "PCRE2 REGULAR EXPRESSION DETAILS"
@ -1671,6 +1671,9 @@ first one in the pattern with the given number. The following pattern matches
.sp .sp
/(?|(abc)|(def))(?1)/ /(?|(abc)|(def))(?1)/
.sp .sp
A relative reference such as (?-1) is no different: it is just a convenient way
of computing an absolute group number.
.P
If a If a
.\" HTML <a href="#conditions"> .\" HTML <a href="#conditions">
.\" </a> .\" </a>
@ -2652,6 +2655,23 @@ 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 parentheses preceding the recursion. In other words, a negative number counts
capturing parentheses leftwards from the point at which it is encountered. capturing parentheses leftwards from the point at which it is encountered.
.P .P
Be aware however, that if
.\" HTML <a href="#dupsubpatternnumber">
.\" </a>
duplicate subpattern numbers
.\"
are in use, relative references refer to the earliest subpattern with the
appropriate number. Consider, for example:
.sp
(?|(a)|(b)) (c) (?-2)
.sp
The first two capturing groups (a) and (b) are both numbered 1, and group (c)
is number 2. When the reference (?-2) is encountered, the second most recently
opened parentheses has the number 1, but it is the first such group (the (a)
group) to which the recursion refers. This would be the same if an absolute
reference (?1) was used. In other words, relative references are just a
shorthand for computing a group number.
.P
It is also possible to refer to subsequently opened parentheses, by writing It is also possible to refer to subsequently opened parentheses, by writing
references such as (?+2). However, these cannot be recursive because the references such as (?+2). However, these cannot be recursive because the
reference is not inside the parentheses that are referenced. They are always reference is not inside the parentheses that are referenced. They are always
@ -3393,6 +3413,6 @@ Cambridge, England.
.rs .rs
.sp .sp
.nf .nf
Last updated: 01 November 2015 Last updated: 10 November 2015
Copyright (c) 1997-2015 University of Cambridge. Copyright (c) 1997-2015 University of Cambridge.
.fi .fi