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Re: Re: using a hash to do substitution?

by Henri Icarus (Beadle)
on Jun 14, 2001 at 05:36 UTC ( #88273=note: print w/replies, xml ) Need Help??


in reply to Re: using a hash to do substitution?
in thread using a hash to do substitution?

no_slogan's regex trick is very nice (++), however, I've been led to believe that lots of ors (the | char) in a regex can blow out the stack (or take a long time) because of the back tracking required. Can any more adept monks comment on this?

The other more brute force less pretty way to solve the problem is:

foreach $key (keys %hash) { $text =~ s/$key/$hash{$key}/eg; }

-I went outside... and then I came back in!!!!

Replies are listed 'Best First'.
Re: Re: Re: using a hash to do substitution?
by tachyon (Chancellor) on Jun 14, 2001 at 11:02 UTC

    Rather than speculate on efficiency you can always use benchmark. One small point though - whenever you interpolate a string into a m// regex or the first half of a s/// regex you need to backslash your special regex metacharacters $^*()+{[\|.?

    The easiest way is to use quotemeta.

    foreach $key (keys %hash) { $key = quotemeta $key; $text =~ s/$key/$hash{$key}/eg; }

    This is *vital* for reliability. Otherwise you will get unexpected runtime failures when your data eventually contains metachars (typos, deliberate, malicious...)

    use Benchmark; timethese(10000, { 'Simple loop' => ' $text = "This is my test string"; %hash = qw(test text foo bar use loop the end); foreach $key (keys %hash) { $key = quotemeta $key; $text =~ s/$key/$hash{$key}/eg; } ', 'Alternation' => ' $text = "This is my test string"; %hash = qw(test text foo bar use loop the end); $regex = join("|", map(quotemeta, keys %hash)); $text =~ s/\b($regex)\b/$hash{$1}/eg; ', } ); Output: Benchmark: timing 100000 iterations of Alternation, Simple loop... Alternation: 5 wallclock secs ( 4.12 usr + 0.00 sys = 4.12 CPU) @ 2 +4271.84/s (n=100000) Simple loop: 10 wallclock secs ( 9.72 usr + 0.00 sys = 9.72 CPU) @ 1 +0288.07/s (n=100000)

    So as it happens alternation is twice as fast. I thought your solution would be faster but there you go!

    If in doubt - use Benchmark

    Cheers

    tachyon

Re: Re: Re: using a hash to do substitution?
by MeowChow (Vicar) on Jun 14, 2001 at 11:12 UTC
    On the other hand, an alternating regex can use the /o modifier, while your version can't. Regex compilation is a costly operation, so alternation should be much faster under /o.
       MeowChow                                   
                   s aamecha.s a..a\u$&owag.print

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