You are correct that the pattern .*?foo can be written as (?:[^f]+|f(?!oo))*foo, or the variant you showed. You are correct in assuming that this general principle can be used for practically any regex.

However, you should "unroll the loop" of that regex. Look at these results:

use Benchmark 'cmpthese';
my $str = "this is the best end";
cmpthese(-5, {
  extra => sub { $str =~ /(?:[^e]+|e(?!nd))*end/ },
  plain => sub { $str =~ /.*?end/ },
});
__END__
     extra:  12019.37/s (n=60818)
     plain:  49615.59/s (n=283305)

         Rate extra plain
extra 12019/s    --  -76%
plain 49616/s  313%    --
[download]

If we unroll the loop, we get better results.

use Benchmark 'cmpthese';
my $str = "this is the best end";
cmpthese(-5, {
  extra => sub { $str =~ /(?:[^e]+|e(?!nd))*end/ },
  plain => sub { $str =~ /.*?end/ },
  uroll => sub { $str =~ /[^e]*(?:e(?!nd)[^e]*)*end/ },
});
__END__
     extra:  11711.62/s (n=61486)
     plain:  49329.33/s (n=250593)
     uroll:  17985.61/s (n=94964)

         Rate extra uroll plain
extra 11712/s    --  -35%  -76%
uroll 17986/s   54%    --  -64%
plain 49329/s  321%  174%    --
[download]

But the point is, the internal optimization uses less regex op-codes, and so it is faster -- it has the engine do the work itself, it doesn't make the regex do the work.

_____________________________________________________
Jeff[japhy]Pinyan: Perl, regex, and perl hacker, who'd like a (from-home) job
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;

---

---

• Are you posting in the right place? Check out Where do I post X? to know for sure.
• Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:

  <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>

• Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
• Want more info? How to link or How to display code and escape characters are good places to start.