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Using variables in regex search

by lancer (Beadle)
on Aug 14, 2011 at 13:07 UTC ( #920222=perlquestion: print w/ replies, xml ) Need Help??
lancer has asked for the wisdom of the Perl Monks concerning the following question:

Hi All!

I'm trying to find a string in a text, and the string is coming from a variable.

What expression should I use?

Will this code work?

$text =~ m/$str/;
Will I need to escape the contents of "$str" ?

Thank you

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Re: Using variables in regex search
by Tanktalus (Canon) on Aug 14, 2011 at 13:15 UTC
    If you're looking for a literal string, use index instead of a regex. If the string is a regex itself, then what you have is fine.
      Thanks, Tanktalus!

      I'm looking for a literal string. But I would like to find all occurrences of it, not just the first one (index gives only the first match).

      A regex search could iterate over all matches. But I think I have to escape the contents of $str, because it's not a regex expression itself...

        Now it's sounding like an XY Problem. What do you need to do with all found occurrences? Modify them? With a literal string, I can only think of three things to do with it: check if it's there (boolean - a single match is sufficient), modify it (s/../../), or count it. And the vast majority IME is the first one. Only the modification one "needs" a regex, and even that isn't really true.

        That all said, index can find multiple matches as well:

        $ perl -lE ' my ($haystack,$needle)=@ARGV; my $i=0; my @found; while(-1 != (my $curidx = index $haystack, $needle, $i)) { push @found, $curidx; $i = $curidx+1 }; say "found at $_" for @found ' abcsdfabcasegabc abc found at 0 found at 6 found at 13
        If you're doing a modification, just use rindex - it's even easier, use rindex (though this will be a bit slower for longer strings with many matches).
        $ perl -lE ' my ($haystack,$needle,$new)=@ARGV; while(-1 != (my $curidx = rindex $haystack, $needle)) { substr $haystack, $curidx, length($needle), $new }; say "new string: $haystack" ' abcsdfabcasegabc abc foo new string: foosdffooasegfoo
        The only challenge with this method is if $new contains $needle in it - then it won't work.

        If you are going with the substitution and want to use a regex (probably safer once you escape it), use "\Q" before your string:

        s/\Q$str\E/$new/g; # since \E is at the end, it's not really required.
        Hope that helps.

Re: Using variables in regex search
by toolic (Chancellor) on Aug 14, 2011 at 16:58 UTC

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