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.

3 files changed, 57 insertions, 4 deletions

diff --git a/pod/perldelta.pod b/pod/perldelta.pod
index ce1a2adec5..4183474c53 100644
--- a/pod/perldelta.pod
+++ b/pod/perldelta.pod
@@ -43,6 +43,25 @@ have a constant target label, and that label is found within the block.
     LABEL: print "This does\n";
   }
 
+=head2 New regexp variable ${^LAST_SUCCESSFUL_PATTERN}
+
+This allows access to the last succesful pattern that matched in the current scope.
+Many aspects of the regex engine refer to the "last successful pattern". The empty
+pattern reuses it, and all of the magic regex vars relate to it. This allows
+access to its pattern. The following code
+
+    if (m/foo/ || m/bar/) {
+        s//PQR/;
+    }
+
+can be rewritten as follows
+
+    if (m/foo/ || m/bar/) {
+        s/${^LAST_SUCCESSFUL_PATTERN}/PQR/;
+    }
+
+and it will do the exactly same thing.
+
 =head1 Security
 
 XXX Any security-related notices go here.  In particular, any security
diff --git a/pod/perlop.pod b/pod/perlop.pod
index 35b2fe4d7b..07c01a5279 100644
--- a/pod/perlop.pod
+++ b/pod/perlop.pod
@@ -2055,11 +2055,24 @@ The bottom line is that using C</o> is almost never a good idea.
 =item The empty pattern C<//>
 
 If the I<PATTERN> evaluates to the empty string, the last
-I<successfully> matched regular expression is used instead.  In this
-case, only the C<g> and C<c> flags on the empty pattern are honored;
-the other flags are taken from the original pattern.  If no match has
+I<successfully> matched regular expression is used instead. In this
+case, only the C<g> and C<c> flags on the empty pattern are honored; the
+other flags are taken from the original pattern. If no match has
 previously succeeded, this will (silently) act instead as a genuine
-empty pattern (which will always match).
+empty pattern (which will always match).  Using a user supplied string as
+a pattern has the risk that if the string is empty that it triggers the
+"last successful match" behavior, which can be very confusing. In such
+cases you are recommended to replace C<m/$pattern/> with
+C<m/(?:$pattern)/> to avoid this behavior.
+
+The last successful pattern may be accessed as a variable via
+C<${^LAST_SUCCESSFUL_PATTERN}>. Matching against it, or the empty
+pattern should have the same effect, with the exception that when there
+is no last successful pattern the empty pattern will silently match,
+whereas using the C<${^LAST_SUCCESSFUL_PATTERN}> variable will produce
+undefined warnings (if warnings are enabled). You can check
+C<defined(${^LAST_SUCCESSFUL_PATTERN})> to test if there is a "last
+successful match" in the current scope.
 
 Note that it's possible to confuse Perl into thinking C<//> (the empty
 regex) is really C<//> (the defined-or operator).  Perl is usually pretty
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