Regexes that use a string as basis for matching

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

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Re: Regexes that use a string as basis for matching by holli (Abbot) on Jun 14, 2008 at 21:59 UTC
you mean like `$s = "match"; $re = qr/$s/; print "yup" if $something =~ $re;` [download] or even simpler (but slower) `$s = "match"; print "yup" if $something =~ /$s/;` [download] holli, /regexed monk/	[reply] [d/l] [select]
Re^2: Regexes that use a string as basis for matching by ikegami (Patriarch) on Jun 15, 2008 at 05:35 UTC
The second snippet shouldn't be any slower. In both snippets, the regexp is compiled once. However, `my @regexps = ( 'pattern1', 'pattern2', ); for my $s (@strings) { for $re (@regexps) { something($s =~ /$re/); } }` [download] is slower than `my @regexps = ( qr/pattern1/, qr/pattern2/, ); for my $s (@strings) { for $re (@regexps) { something($s =~ /$re/); } }` [download] since the former compiles each regexp for each string, while the latter compiles each regexp once.	[reply] [d/l] [select]
Re^2: Regexes that use a string as basis for matching by Your Mother (Archbishop) on Jun 15, 2008 at 04:39 UTC
or even simpler (but slower) Why is it slower?	[reply]
Re^2: Regexes that use a string as basis for matching by gatito (Novice) on Jun 14, 2008 at 22:05 UTC
oooh thanks, that worked perfectly	[reply]
Re: Regexes that use a string as basis for matching by ikegami (Patriarch) on Jun 15, 2008 at 00:16 UTC
If `$pattern_str` is plain text and not a regular expression, you'll need to convert/escape it before you can use it as a regular expression. `$str =~ /\Q$text\E/` [download] or `my $re = quotemeta($text); $str =~ /$re/` [download] or `index($str, $text) >= 0` [download] References: perlre (for regular expressions in general and `\Q..\E` specifically) `quotemeta` `index`	[reply] [d/l] [select]
Re: Regexes that use a string as basis for matching by FunkyMonk (Bishop) on Jun 15, 2008 at 00:05 UTC
Matching against a variable is mentioned in perlre and is the fourth example in perlre. Perhaps the official documentation is where you should have looked first Unless I state otherwise, all my code runs with strict and warnings	[reply]