The CPAN module Regexp::MatchContext was written for this task. The module's SYNOPSIS: 
    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]
Note that this and the previous solutions are significantly slower than using the matchvariables &`, $& and $'. However, as tye mentioned, these variables will slow down EVERY other regular expression without capturing parentheses.

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]
Note that this benchmark uses match variables and thus slows down all four solutions. The results without the match variable solution are: 
            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]

In reply to Re: How do I get what is to the left of my match? by lima1
in thread How do I get what is to the left of my match? by NotProud

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