in reply to Regex with variables

Yep, switch the variables in your match test.

But if your data are really like that then you should have them in a hash and check them there rather than stringifying and testing with a regexp. 

#!/usr/bin/env perl
use strict;
use warnings;
my %hash;
for(qw/1689 9679 6978 2792 2514 5472 1520 9342 5544 1268 0165 1979 731
+4 2101 7221 
+9539 3882 1812/) {
  $hash{$_}++;
}
my $tested = '2101';
if( exists $hash{$tested} ) {
    print "OK";
}
else {
    print "Not there";
}

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Re^2: Regex with variables
by stevieb (Canon) on Jun 29, 2015 at 14:19 UTC

    If there are numerous lookups, I agree, but if it's just a couple of quick lookups, that's not necessarily true. To extract the string into a hash seems to take significantly longer than just a straight regex lookup:

    #!/usr/bin/perl

use warnings;
use strict;

use Benchmark qw(:all);

cmpthese ( 5000000, {
                'string' => 'string()',
                'hash' => 'hash()',
            });

sub string {
    my $str = "1689 9679 2792 2514 5472 1520 9342 5544 1268 0165 1979 
+7314 2101 7221 9539 3882 1812";
    my $test = 2101;
    if ($str =~ $test){
        # do stuff
    }
}
sub hash {
    my $str = "1689 9679 2792 2514 5472 1520 9342 5544 1268 0165 1979 
+7314 2101 7221 9539 3882 1812";
    my $test = 2101;
    my %hash;
    for (split /\s+/, $str){
        $hash{$_}++;
    }
    if (exists $hash{$test}){
        # do stuff
    }
}

__END__
            Rate   hash string
hash    267237/s     --   -92%
string 3521127/s  1218%     --

    [download]
[reply]
[d/l]

      Hi Stevieb,

      I wasn't considering performance but rather the ease of maintenance of a list of items. The OP has $targets (plural) as the name of his string, so I assumed that at some point he had a list, and then put it into a string.

      I agree that if he originally had a string like that (e.g. from a log file) then a regexp would be the way to find a substring of it.

      But if he has a list then putting it into a string and looking for a match is headed in the wrong direction, IMO.

      Update: I wanted to know, so now I do. In this case join()ing the list into a string and then using a regexp is still much faster than populating a hash: 

      use Benchmark qw(:all);

cmpthese ( 5000000, {
  'string' => 'string()',
  'hash' => 'hash()',
  'list2string' => 'list2string()',
});

# other subs as before

sub list2string {
  my $str = join( ' ', qw/1689 9679 2792 2514 5472 1520 9342 5544 1268
+ 0165 1979 7314 2101 7221 9539 3882 1812/ );
  my $test = 2101;
  if ($str =~ $test){
    # do stuff
  }
}

__END__

                 Rate                         hash list2string      st
+ring
                 hash          62406/s          --        -88%        
+-95%
                 list2string  530223/s        750%          --        
+-61%
                 string      1355014/s       2071%        156%        
+  --

      [download]

      But I still wouldn't do it myself as combining discrete values into a string just feels wrong.

[reply]
[d/l]
[select]

        If the OP's data was indeed in a list, I'd probably opt for grep(), if grep(/$test/, @list).... The good thing about Perl, is TIMTOWTDI ;)

[reply]
[d/l]
Re^2: Regex with variables
by cormanaz (Deacon) on Jun 29, 2015 at 14:09 UTC
    Doh! I told you it was a stupid question. Thanks for the replies.
[reply]

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