onion2k has asked for the wisdom of the Perl Monks concerning the following question:

I has a minor problem. I want to test if any of a series of regular expressions matches a string. If there is a match I want to print the expression. I achieved this fairly easily using a foreach.. 
    foreach $pattern (@patterns)) {
        if ($input =~ /$pattern/) {
            print $pattern;
        }
    }

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However, commonsense tells me that if theres a large number of patterns thats going to be really slow. Is there, in the classic Perl sense, another way to do it?
Chris
$live{$free}||die

Replies are listed 'Best First'.
Re: More than one regexp.
by danger (Priest) on Jan 19, 2001 at 19:51 UTC

    Well, most people have pointed out the qr// operator, but didn't notice that you wanted to print your re's if matched, not the input. However, you can still speed up your matching by using qr//: 

    #!/usr/bin/perl -w
use strict;
my @patterns = qw/ ^foo
                   bar
                   x.*?z
                 /;
@patterns = map{qr/$_/}@patterns;
my $input = 'fooxbaz';
foreach my $re (@patterns){
    print "$re\n" if $input =~ /$re/;
}

    [download]

    The re's you get back aren't exactly the same as the pattern strings originally provided (notably they'll have '(?-ixsm)' appended to them to indicate which modifiers were present (or not in this case) when compiled).

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[d/l]
Re: More than one regexp.
by I0 (Priest) on Jan 19, 2001 at 15:27 UTC
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Re: More than one regexp.
by RatArsed (Monk) on Jan 19, 2001 at 14:34 UTC
    My feeling would be to try something like: 
    my $patternlist = join( "\t", @patterns );

if( $patternlist =~ /\t$input(.*)\t/ )
{
   print $input.$1;
}

    [download]
    NB This will do something akin to "command matching"

    If you wanted to match whole patterns only, then: 

    my $patternlist = join( "\t", @patterns );

if( $patternlist =~ /\t$input\t/ )
{
   print $input;
}

    [download]
    of course RatArsed
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[d/l]
[select]

      The problem with this is that you have to compile a new regular expression for every line of $input. You can do this sort of alternation in a regular expression like 

      my @patterns = qw/ test good /;
my $alt = join('|', @patterns);
my $re  = qr/(?:$alt)/;

while (<DATA>) {
        if ( /$re/ ) { print "+ $_"; }
        else { print "- $_"; }
}
__END__
THis is a test
this is only a test
now is the time for all
good men to come to the
aid of their country.

      [download]

      Other than perlop and perlre, a good place to look for regular expressions is the Jeffery Friedl book, Mastering Regular Expressions.

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[d/l]
        Assuming that your target strings don't change, tack a /o onto the assembled regexp invocation to compile it once. (See perlre for details.)
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[d/l]
Re: More than one regexp.
by kschwab (Vicar) on Jan 19, 2001 at 18:46 UTC
    You may also want to have a look at study
[reply]

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