Revision: 423
http://www.exim.org/viewvc/pcre2?view=rev&revision=423
Author: ph10
Date: 2015-11-10 14:50:51 +0000 (Tue, 10 Nov 2015)
Log Message:
-----------
Clarify relative refence semantics.
Modified Paths:
--------------
code/trunk/doc/pcre2pattern.3
Modified: code/trunk/doc/pcre2pattern.3
===================================================================
--- code/trunk/doc/pcre2pattern.3 2015-11-10 14:33:28 UTC (rev 422)
+++ code/trunk/doc/pcre2pattern.3 2015-11-10 14:50:51 UTC (rev 423)
@@ -1,4 +1,4 @@
-.TH PCRE2PATTERN 3 "01 November 2015" "PCRE2 10.21"
+.TH PCRE2PATTERN 3 "10 November 2015" "PCRE2 10.21"
.SH NAME
PCRE2 - Perl-compatible regular expressions (revised API)
.SH "PCRE2 REGULAR EXPRESSION DETAILS"
@@ -1671,6 +1671,9 @@
.sp
/(?|(abc)|(def))(?1)/
.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
.\" HTML <a href="#conditions">
.\" </a>
@@ -2652,6 +2655,23 @@
parentheses preceding the recursion. In other words, a negative number counts
capturing parentheses leftwards from the point at which it is encountered.
.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
references such as (?+2). However, these cannot be recursive because the
reference is not inside the parentheses that are referenced. They are always
@@ -3393,6 +3413,6 @@
.rs
.sp
.nf
-Last updated: 01 November 2015
+Last updated: 10 November 2015
Copyright (c) 1997-2015 University of Cambridge.
.fi
