Document backreferences to groups that did not match

Also add a test for that, fill in test description, and sneak in a vim
modeline for re_tests

diff --git a/pod/perlre.pod b/pod/perlre.pod
index df627ff..42017dd 100644 (file)
--- a/pod/perlre.pod
+++ b/pod/perlre.pod
@@ -518,6 +518,10 @@ backreference only if at least 11 left parentheses have opened
 before it.  And so on.  \1 through \9 are always interpreted as
 backreferences.
 
+If the bracketing group did not match, the associated backreference won't
+match either. (This can happen if the bracketing group is optional, or
+in a different branch of an alternation.)
+
 X<\g{1}> X<\g{-1}> X<\g{name}> X<relative backreference> X<named backreference>
 In order to provide a safer and easier way to construct patterns using
 backreferences, Perl provides the C<\g{N}> notation (starting with perl

diff --git a/t/re/re_tests b/t/re/re_tests
index 725a752..dc03084 100644 (file)
--- a/t/re/re_tests
+++ b/t/re/re_tests
@@ -275,7 +275,8 @@ a[-]?c      ac      y       $&      ac
 \g{0}  -       c       -       Reference to invalid group 0
 \g{-0} -       c       -       Reference to invalid group 0
 (a)|\1 a       y       -       -
-(a)|\1 x       n       -       -
+(a)|\1 x       n       -       Reference to group in different branch
+(?:(b)?a)\1    a       n       -       Reference to group that did not match
 (a)|\2 -       c       -       Reference to nonexistent group
 (([a-c])b*?\2)*        ababbbcbc       y       $&-$1-$2        ababb-bb-b
 (([a-c])b*?\2){3}      ababbbcbc       y       $&-$1-$2        ababbbcbc-cbc-c
@@ -1393,3 +1394,5 @@ foo(\h)bar        foo\tbar        y       $1      \t
 '[\x{100}\xff]'i       \x{ff}  y       $&      \x{ff}
 
 ((??{ "(?:|)" }))\s    C\x20   y       -       -
+
+# vim: set noexpandtab