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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In reply to Re^3: Surprisingly poor regex performance
by dragonchild
in thread Surprisingly poor regex performance
by sgifford
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