in reply to Re^2: Surprisingly poor regex performance
in thread Surprisingly poor regex performance

Well, ^ doesn't mean the same as \n? at all. Like not even close.

According to Programming Perl 3rd edition pages 150 and 159, ^, when used with the /m modifier, means to match after embedded newlines or the beginning of the string.

\n? means something akin to possibly match a newline, but maybe not. That doesn't provide the engine with any understanding of where to match. Had you said something like /\n(.*$pat.*\n)/, you would have been better off (except for not matching the first line). Of course, with /m, you should be able to do /^(.*$pat.*)$/omsg and it should be about as efficient as my regex.

With regexes, it's always better to be as explicit as possible. This will allow the engine to make a number of optimizations. Some of those optimizations, as you have found out, can mean the difference between 2500 seconds and 23 seconds.

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Re^4: Surprisingly poor regex performance
by sgifford (Prior) on Dec 13, 2004 at 23:30 UTC
    You're right of course; I shouldn't have said meant the same as; I meant would have the same effect as.

    Still, I think it's accurate to say that ^ means very nearly the same as your example: (?:\A|\n), so it's still quite surprising to me that yours is so much faster.

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      I'm actually really curious about that, as well. I /msg'ed japhy asking if he'd pop in and help us out. My benchmarking shows a 15x speedup using (?:\A|\n) over using ^ with the /m modifier.

      Oh - taking away the /m modifier when using (?:\A|\n) results in potentially something like a 1% speedup. I guess randomly adding modifiers is bad. :-)

      Being right, does not endow the right to be rude; politeness costs nothing.
      Being unknowing, is not the same as being stupid.
      Expressing a contrary opinion, whether to the individual or the group, is more often a sign of deeper thought than of cantankerous belligerence.
      Do not mistake your goals as the only goals; your opinion as the only opinion; your confidence as correctness. Saying you know better is not the same as explaining you know better.

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        How does it compare to (?:\A|(?<=\n)) ? Isn't that the accurate representation of /^.../m ? You could also try (?<![^\n]) which seems somehow even simpler.

        - tye        

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