Note that if you ever even mention $`, $&, or $', anywhere, then all regular expressions anywhere in that run of Perl will be not-insignificantly slower. So their use is strongly discouraged in code that might be reused or where performance is important. This is because using those anywhere forces each regex to make copies of those strings every time, even though most of those copies will never be used (if Perl ever needs them then Perl can't predict when it might need them and so must always make the copies).

But the latest version of Perl adds an alternate way to get this type of information, @- and @+. Here is a sample of how to use them:

( my $str= "left center right" ) =~ /center/;
print "\nLeft:  <", substr( $str, 0, $-[0] ),
      ">\nMatch: <", substr( $str, $-[$#-], $+[$#-] - $-[$#-] ),
      ">\nRight: <", substr( $str, $+[$#+] ), ">\n";
__END__
This prints:
Left:  <left >
Match: <center>
Right: < right>
[download]

If you can't find perlvar.pod then enter the command perldoc perlvar or cd to your perl lib directory and there should be a pod directory that contains that file. These pod files contain some simple "mark-up" codes but are designed to be easy for humans to read. (You can read perlpod.pod for more information on the mark-up language.)

Thanks to ar0n for updating the above node. This node is now defunct and should be deleted soon. (:

Nothing to see here. Move along.

        - tye (but my friends call me "Tye")

$_="left center right";
m/(.*)(center)(.*)/;
print "$1:$2:$3\n";
[download]

Keep in mind that /(center)/ will match the first occurence of 'center' in the string, while /(.*)(center)(.*)/ will match the last occurence of center (leaving aside for the moment the issue of newlines).

This might be preferred:

/(.*?)(center)(.*)/s;
[download]

On the other hand, sometimes using $` et al. is appropriate.

    use Regexp::MatchContext -vars;

    $str = m/(?p) \d+ /;

    print "Before:  $PREMATCH\n";
    print "Matched: $MATCH\n";
    print "After:   $POSTMATCH\n";

    $MATCH = 2 * $MATCH;    # substitute into original $str
[download]

The following benchmark (searching a short (11 characters or base pairs) DNA sequence in a 2000 bp DNA sequence) shows the results of a comparison of all four solutions:

              Rate     regex   context  at_minus matchvars
regex      17271/s        --      -22%      -66%      -84%
context    22239/s       29%        --      -56%      -79%
at_minus   50420/s      192%      127%        --      -53%
matchvars 107527/s      523%      384%      113%        --
[download]

            Rate    regex  context at_minus
regex    17544/s       --     -24%     -69%
context  23112/s      32%       --     -60%
at_minus 57361/s     227%     148%       --
[download]

Appendix: Source code of the benchmark

#!/usr/bin/perl
use strict;
use warnings;
use Benchmark qw(:all);
use Regexp::MatchContext;

my $count   = 300000;
# to test that all solutions produce the same output
my $VERBOSE = 0;
$count = 2 if $VERBOSE;

my $seq
    = 'GGGTTGAAGTTTAGACCGCTCACAGTAGTTCTACCTATAGAAAAGATCATGAAAGAGGCGATC
+AGAATGGTACTCGAATCCATTTACGATCCCGAGTTTCCAGACACATCGCATTTCCGCTCGGGTCAAGGC
+TGCCACTCGGTCCTAAGACGGATCAAAGAAGAGTGGGGAATCTCTCGCTGGTTTTTAGAATTCGACATC
+AGGAAGTGTTTTCACACCATCGACCGACATCGACTCATCCAAATTTTGAAGGAAGAGATCGACGATCCC
+AAGTTCTTTTACTCCATTCAGAAAGTATTTTCCGCCGGACGACTCGTAGGAGTTGAGAGGGGCCCTTAC
+TCCGTCCCACACAGTGTACTACTATCGGCCCTACCAGGCAACATCTACCTACACAAGCTCGATCAGGAG
+ATAGGGAGGATCCGACAGAAGTACGAAATTCCGATTGTTCAGAGAGTCAGATCGGTTCTATTAAGGACA
+GGTCGTCGTATTGATGACCAAGAAAACCCTGGAGAAGAAGCAAGCTTCAACGCTCCCCAAGACAACAGA
+GCCATCATTGTGGGGAGCGTTAAGAGCATGCAACGCAAAGCGGCCTTTCATTCCCTTGTTTCGTCGTGG
+CACACCCCCCCCACAAGCACCCTCCGGCTCAGGGGGGACCAGAAAAGGCCTTTCGTTTTCCCCCCTTCG
+TCGGCCCTTGCCGTCTTCCTTAACAAGCCCTCGAGCCTTCTTTGCGCCGCCTTCCTCATAGAAGCCGCC
+GGGTTGACCCCGAAGGCTGAATTCTATGGTGGAGAACGCTGTAATAATAATTGGGCCATGAGAGACCTT
+CTTAAGTATTGCAAAAGAAAGGGCCTGCTGATAGAGCTGGGCGGGGAGGCGATACTAGTTATCAGGTCA
+GAGAGAGGCCTGGCCCGTAAGCAGGCCCCCTTAAAAACCCATTACTTAATAAGGATTTGTTACGCGCGA
+TATGCCGACGACTTACTACTGGGAATCGTGGGTGCCGTAGAGCTTCTCATAGAAATACAAAAACGTATC
+GCCCATTTCCTACAATCTGGCCTGAACCTTTGGGTAGGCTCCGCAGGATCAACAACAATAGCTGCACGG
+AGTACGGTAGAATTCCTTGGTACGGTCATTCGGGAAGTCCCTCCGAGGACGACTCCCATACAATTTTTG
+CGAGAGCTGGAAAAGCGTCTACGGGTAAAGCACCGTATCCATATAACTGCTTGCCACCTACGCTCCGCC
+ATCCATTCAAAGTTTAGGAACCTAGGTGATAGTATCCCGATCAAACAGCTGACGAAGGGGATGAGCAAA
+ACAGGGAGTCTACAGGACGGGGTTCAACTAGCGGAGACTCTTGGAACAGCTGGAGTCAGAAGTCCCCAA
+GTTAGCGTATTATGGGGGACCGTCAAGCACATCCGGCAAGGATCAAGGGGGATCTCGTTCTTGCATAGC
+TCAGGTCGGAGCAACGCGTCATCGGACGTTCAACAGGTAGTCTCACGATCGGGCACTCATGCCCGTAAG
+TTGTCATTGTATACTCCCCCGGGTCGGAAGGCGGCGGGGGAGGGAGGAGGACACTGGGCGGGATCTATC
+AGCAGCGAATTCCCCATAAAGATAGAGGCACCTATAAAAAAGATACTCCGAAGGCTTCGGGATCGAGGT
+ATCATTAGCCGAAGAAGACCCTGGCCAATCCACGTGGCCTGTTTGACGAACGTCAGCGACGAAGACATC
+GTAAATTGGTCCGCGGGCATCGCGATAAGTCCTCTGTCCTACTACAGGTGCCGCGACAACCTTTATCAA
+GTCCGAACGATTGTCGACCACCAGATTCGCTGGTCTGCAATATTCACCCTAGCCCACAAGCACAAATCC
+TCGGCGCCGAATATAATCCTCAAGTACTCCAAAGACTCAAATATTGTAAATCAAGAAGGTGGCAAGATC
+CTTGCAGAGTTCCCCAACAGCATAGAGCTTGGGAAGCTCGGACCCGGTCAAGACCTGAACAAGAAGGAA
+CACTCAACTACTAGTCTAGTCTAG';

cmpthese(
    $count,
    {   'regex' => sub {
            my ( $prematch, $match, $postmatch )
                = $seq =~ m{(\A .*?) (CTGGCCCGTAA) (.*\z) }xms;
            warn "$prematch   $match   $postmatch" if $VERBOSE;
        },
        'matchvars' => sub {
            $seq =~ m{CTGGCCCGTAA}xms;
            my ($prematch, $match, $postmatch) = ($`, $&, $');
            warn "$prematch   $match   $postmatch" if $VERBOSE;
        },
        'context' => sub {
            $seq =~ m{(?p)CTGGCCCGTAA}xms;
            my ( $prematch, $match, $postmatch )
                = ( PREMATCH(), MATCH(), POSTMATCH() );
            warn "$prematch   $match   $postmatch" if $VERBOSE;
        },
        'at_minus' => sub {
            $seq =~ m{CTGGCCCGTAA}xms;
            my $prematch = substr( $seq, 0, $-[0] );
            my $match = substr( $seq, $-[$#-], $+[$#-] - $-[$#-] );
            my $postmatch = substr( $seq, $+[$#+] );
            warn "$prematch   $match   $postmatch" if $VERBOSE;
        },
    }
);
[download]