regcomp.c - add optimistic eval (*{ ... }) and (**{ ... })

This adds (*{ ... }) and (**{ ... }) as equivalents to (?{ ... }) and
(??{ ... }). The only difference being that the star variants are
"optimisitic" and are defined to never disable optimisations. This is
especially relevant now that use of (?{ ... }) prevents important
optimisations anywhere in the pattern, instead of the older and inconsistent
rules where it only affected the parts that contained the EVAL.

It is also very useful for injecting debugging style expressions to the
pattern to understand what the regex engine is actually doing. The older
style (?{ ... }) variants would change the regex engines behavior, meaning
this was not as effective a tool as it could have been.

Similarly it is now possible to test that a given regex optimisation
works correctly using (*{ ... }), which was not possible with (?{ ... }).

1 files changed, 14 insertions, 0 deletions

diff --git a/pod/perldelta.pod b/pod/perldelta.pod
index a4cbfbe4a2..81b99f8630 100644
--- a/pod/perldelta.pod
+++ b/pod/perldelta.pod
@@ -27,6 +27,20 @@ here, but most should go in the L</Performance Enhancements> section.
 
 [ List each enhancement as a =head2 entry ]
 
+=head2 Optimistic Eval in Patterns
+
+The use of C<(?{ ... })> and C<(??{ ... })> in a pattern disables various
+optimizations globally in that pattern. This may or may not be desired by the
+programmer. This release adds the C<(*{ ... })> and C<(**{ ... })>
+equivalents. The only difference is that they do not and will never disable
+any optimisations in the regex engine. This may make them more unstable in the
+sense that they may be called more or less times in the future, however the
+number of times they execute will truly match how the regex engine functions.
+For example, certain types of optmisation are disabled when C<(?{ ... })> is
+included in a pattern, so that patterns which are O(N) in normal use become
+O(N*N) with a C<(?{ ... })> pattern in them. Switching to C<(*{ ... })> means
+the pattern will stay O(N).
+
 =head1 Security
 
 XXX Any security-related notices go here.  In particular, any security