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Re: Apply A Set Of Regexes To A String

by tachyon (Chancellor)
on Oct 11, 2004 at 02:37 UTC ( #398081=note: print w/replies, xml ) Need Help??

in reply to Apply A Set Of Regexes To A String

I would suggest this. You can simplify the hash if you are happy to use the substitution pattern as the top level key. The guts of the approach is to build an alternation RE dynamically and use the match value to lookup the replacement value in a hash. This is typically the fastest approach as you leverage the C code in the regex and hashing engines effectively. Note the sort by longest first so we match on the full 'foobar' not 'foo' or 'bar'.

my $res = { re1 => { foo => 'foo_new' }, re2 => { bar => 'bar_new' }, re3 => { qux => 'bar_new' }, re4 => { foobar => 'foobar_new' } }; my @required = qw ( re1 re2 re4 ); my %active_re = map{ each %{$res->{$_}} } @required; my $match = join '|', sort{ length $b <=> length $a } keys %active_re; $match = qr/($match)/; $str = 'foo bar baz foobar'; $str =~ s/$match/$active_re{$1}/g; print $str;



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Re^2: Apply A Set Of Regexes To A String
by Cody Pendant (Prior) on Oct 12, 2004 at 00:22 UTC
    Thanks Tachyon, I think I'm going to go with something like that solution.

    If you've got a moment, can you say just a little more about why it would be particularly efficient, with regard to the "C code in the regex and hashing engines"? Thanks.

    =~y~b-v~a-z~s; print

      One of the main benefits of alternation is that you can compile the RE. Essentially an alternation RE is very similar to the loop, but the loop code has been optimised to the task and is in C with alternation, but generalised and less efficient if you do it in perl. The difference is significant, at least that is what this Benchmark shows.....

      use Benchmark 'cmpthese'; $iterations = 1000000; %re = ( foo => 'foo1', bar => 'bar1', ); $re = join '|', keys %re; $re = qr/($re)/; $name1 = "RE"; $code1 = << 'END_CODE1'; $_ = 'foo bar'; s/$re/$re{$1}/g; END_CODE1 $name2 = "Loop"; $code2 = << 'END_CODE2'; $_ = 'foo bar'; for my $sub( keys %re ) { s/$sub/$re{$sub}/g; } END_CODE2 cmpthese( $iterations, {$name1 => $code1, $name2 => $code2} ); __END__ Benchmark: timing 1000000 iterations of Loop, RE... Loop: 24 wallclock secs (24.77 usr + 0.00 sys = 24.77 CPU) @ 40 +377.94/s (n=1000000) RE: 8 wallclock secs ( 7.27 usr + 0.00 sys = 7.27 CPU) @ 13 +7551.58/s (n=1000000) Rate Loop RE Loop 40378/s -- -71% RE 137552/s 241% --

      But if I change that to a more real world situation by making the string a 14Kb one (approximately a web page size)

      $_ = 'foo bar' x 2000; Benchmark: timing 10000 iterations of Loop, RE... Loop: 20 wallclock secs (19.90 usr + 0.00 sys = 19.90 CPU) @ 50 +2.56/s (n=10000) RE: 30 wallclock secs (29.44 usr + 0.00 sys = 29.44 CPU) @ 33 +9.64/s (n=10000) Rate RE Loop RE 340/s -- -32% Loop 503/s 48% -

      And now the loop is faster. In fact try this case:

      $_ = 'fo ba' x 1000 . 'foo bar'; Benchmark: timing 10000 iterations of Loop, RE... Loop: 1 wallclock secs ( 0.71 usr + 0.00 sys = 0.71 CPU) @ 14 +064.70/s (n=10000) RE: 12 wallclock secs (11.10 usr + 0.00 sys = 11.10 CPU) @ 90 +1.23/s (n=10000) Rate RE Loop RE 901/s -- -94% Loop 14065/s 1461% --

      ~This is a purpose designed worst case for alternation as it requires continouous back tracking. So I have shattered my own delusions! Perl loops are faster than RE alternation.



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