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| -rw-r--r-- | pod/perlretut.pod | 33 |
1 files changed, 33 insertions, 0 deletions
diff --git a/pod/perlretut.pod b/pod/perlretut.pod index 87669e50ab..2c449f8b07 100644 --- a/pod/perlretut.pod +++ b/pod/perlretut.pod @@ -2046,8 +2046,41 @@ in the regexp. Here are some silly examples: # prints 'Hi Mom!' $x =~ /aaa(?{print "Hi Mom!";})def/; # doesn't match, # no 'Hi Mom!' + +Pay careful attention to the next example: + $x =~ /abc(?{print "Hi Mom!";})ddd/; # doesn't match, # no 'Hi Mom!' + # but why not? + +At first glance, you'd think that it shouldn't print, because obviously +the C<ddd> isn't going to match the target string. But look at this +example: + + $x =~ /abc(?{print "Hi Mom!";})[d]dd/; # doesn't match, + # but _does_ print + +Hmm. What happened here? If you've been following along, you know that +the above pattern should be effectively the same as the last one -- +enclosing the d in a character class isn't going to change what it +matches. So why does the first not print while the second one does? + +The answer lies in the optimizations the REx engine makes. In the first +case, all the engine sees are plain old characters (aside from the +C<?{}> construct). It's smart enough to realize that the string 'ddd' +doesn't occur in our target string before actually running the pattern +through. But in the second case, we've tricked it into thinking that our +pattern is more complicated than it is. It takes a look, sees our +character class, and decides that it will have to actually run the +pattern to determine whether or not it matches, and in the process of +running it hits the print statement before it discovers that we don't +have a match. + +To take a closer look at how the engine does optimizations, see the +section L<"Pragmas and debugging"> below. + +More fun with C<?{}>: + $x =~ /(?{print "Hi Mom!";})/; # matches, # prints 'Hi Mom!' $x =~ /(?{$c = 1;})(?{print "$c";})/; # matches, |