Try this snippet:

#!/usr/bin/perl -w
use strict;

use re 'debug';
my $test = "abc123abc1234abc";

$test =~ /(\d{4})/;
print $1;
[download]

The output from that is:

Compiling REx `(\d{4})'
size 8 first at 5
   1: OPEN1(3)
   3:   CURLY {4,4}(6)
   5:     DIGIT(0)
   6: CLOSE1(8)
   8: END(0)
stclass `DIGIT' minlen 4
Matching REx `(\d{4})' against `abc123abc1234abc'
  Setting an EVAL scope, savestack=5
   3 <abc> <123abc123>    |  1:  OPEN1
   3 <abc> <123abc123>    |  3:  CURLY {4,4}
                           DIGIT can match 3 times out of 4...
  Setting an EVAL scope, savestack=5
                            failed...
  Setting an EVAL scope, savestack=5
   4 <abc1> <23abc123>    |  1:  OPEN1
   4 <abc1> <23abc123>    |  3:  CURLY {4,4}
                           DIGIT can match 2 times out of 4...
  Setting an EVAL scope, savestack=5
                            failed...
  Setting an EVAL scope, savestack=5
   5 <abc12> <3abc123>    |  1:  OPEN1
   5 <abc12> <3abc123>    |  3:  CURLY {4,4}
                           DIGIT can match 1 times out of 4...
  Setting an EVAL scope, savestack=5
                            failed...
  Setting an EVAL scope, savestack=5
   9 <23abc> <1234abc>    |  1:  OPEN1
   9 <23abc> <1234abc>    |  3:  CURLY {4,4}
                           DIGIT can match 4 times out of 4...
  Setting an EVAL scope, savestack=5
  13 <23abc1234> <abc>    |  6:    CLOSE1
  13 <23abc1234> <abc>    |  8:    END
Match successful!
1234Freeing REx: `(\d{4})'
[download]

It's fairly complicated, but if you really need to get into the guts of how regular expressions work, that should do it.

Cheers,
Ovid

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-RT

There's also Dominus' Rx

Which is based on Rx. Dominus wrote an excellent paper on the development of Rx. It won a "best paper" award at The Perl Conference this year. It's included in the proceedings of the conference or on his web site.

--
<http://www.dave.org.uk>

"The first rule of Perl club is you don't talk about Perl club."