When you have code like this 

# we read a line from somewhere to $line
# and @regex is our array of patterns
foreach (@regex) {
    print "Match!\n" if $line=/$_/;
}

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Each time the regex is run it has a different pattern so has to compile the pattern again. I belive compiling the pattern can be quite costly for complex regexen. However what I have done is first to expand all the patterns into a code block 
# {
#    my @matches;
#    push @matches 1 if /first_regex/;
#    push @matches 2 is /second_regex/;
#      .
#      .
#    push @matches n if /nth_regex/;
#    @matches;
# }

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This code block is stored in a scalar and eval'd. It runs against the line stored in $_ and returns a list of indices telling us which regex was matched. Now Perl can see that it has a series of invariant regexen and after the first eval caches the compiled regexen and uses this version on subsequent itterations. the problem is still n*m omplexity but we have stopped perl doing rather a large amount of work in each itteration. Certainly for my problem here comparing over 400 lines of log to over 600 reasonably complex regexen the speed up was >10 times.

Cheers,
R.

In reply to Re^4: Matching against list of patterns by Random_Walk
in thread Matching against list of patterns by Eyck

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