Forbid \g0. (tests coming later)

p4raw-id: //depot/perl@31524

2 files changed, 9 insertions, 0 deletions

diff --git a/pod/perldiag.pod b/pod/perldiag.pod
index d0cedfc28a..338fd12da6 100644
--- a/pod/perldiag.pod
+++ b/pod/perldiag.pod
@@ -3556,6 +3556,13 @@ Doing so has no effect.
 (W internal) The internal sv_replace() function was handed a new SV with
 a reference count of other than 1.
 
+=item Reference to invalid group 0
+
+(F) You used C<\g0> or similar in a regular expression. You may refer to
+capturing parentheses only with strictly positive integers (normal
+backreferences) or with stricly negative integers (relative
+backreferences), but using 0 does not make sense.
+
 =item Reference to nonexistent group in regex; marked by <-- HERE in m/%s/
 
 (F) You used something like C<\7> in your regular expression, but there are
diff --git a/regcomp.c b/regcomp.c
index 60b31ed23b..cada4cdd26 100644
--- a/regcomp.c
+++ b/regcomp.c
@@ -7052,6 +7052,8 @@ tryagain:
 		        goto parse_named_seq;
 		}   }
 		num = atoi(RExC_parse);
+		if (isg && num == 0)
+		    vFAIL("Reference to invalid group 0");
                 if (isrel) {
                     num = RExC_npar - num;
                     if (num < 1)