mg.c - add support for ${^LAST_SUCCESSFUL_PATTERN}

This exposes the "last successful pattern" as a variable that can be
printed, or used in patterns, or tested for definedness, etc. Many regex
magical variables relate to PL_curpm, which contains the last successful
match.  We never exposed the *pattern* directly, although it was
implicitly available via the "empty pattern".  With this patch it is
exposed explicitly.  This means that if someone embeds a pattern as a
match operator it can then be accessed after the fact much like a qr//
variable would be.

@ether asked if we had this, and I had to say "no", which was a shame as
obviously the code involved isn't very complicated (the docs from this
patch are far larger than the code involved!).  At the very least
this can be useful for debugging and probably testing. It can also
be useful to test if the /is/ a "last successful pattern", by checking
if the var is defined.

1 files changed, 21 insertions, 0 deletions

diff --git a/pod/perlvar.pod b/pod/perlvar.pod
index 565d0206f5..aebd6a45bd 100644
--- a/pod/perlvar.pod
+++ b/pod/perlvar.pod
@@ -1372,6 +1372,27 @@ added in 5.25.7.
 
 This variable is read-only, and its value is dynamically scoped.
 
+=item ${^LAST_SUCCESSFUL_PATTERN}
+
+The last successful pattern that matched in the current scope.  The empty
+pattern defaults to matching to this. For instance:
+
+    if (m/foo/ || m/bar/) {
+        s//BLAH/;
+    }
+
+and
+
+    if (m/foo/ || m/bar/) {
+        s/${^LAST_SUCCESSFUL_PATTERN}/BLAH/;
+    }
+
+are equivalent. 
+
+You can use this to debug which pattern matched last, or to match with it again.
+
+Added in Perl 5.37.10.
+
 =item $LAST_REGEXP_CODE_RESULT
 
 =item $^R