perl -Dx somescript.pl ##

##

    use strict;
    use Data::Dumper;
    use re 'debug';

    'abc123def456' =~ /(?<=f)(\d+)/;
    print "$1\n";

##

##

Compiling REx `(?<=f)(\d+)'
size 13 first at 1
synthetic stclass `ANYOF[0-9]'.
   1: IFMATCH[-1](7)
   3:   EXACT (5)
   5:   SUCCEED(0)
   6:   TAIL(7)
   7: OPEN1(9)
   9:   PLUS(11)
  10:     DIGIT(0)
  11: CLOSE1(13)
  13: END(0)
stclass `ANYOF[0-9]' minlen 1
Matching REx `(?<=f)(\d+)' against `abc123def456'
  Setting an EVAL scope, savestack=3
   3  <123def456>    |  1:  IFMATCH[-1]
   2      |  3:    EXACT 
                              failed...
                            failed...
  Setting an EVAL scope, savestack=3
   4  <23def456>    |  1:  IFMATCH[-1]
   3  <123def456>    |  3:    EXACT 
                              failed...
                            failed...
  Setting an EVAL scope, savestack=3
   5  <3def456>    |  1:  IFMATCH[-1]
   4  <23def456>    |  3:    EXACT 
                              failed...
                            failed...
  Setting an EVAL scope, savestack=3
   9  <456>    |  1:  IFMATCH[-1]
   8      |  3:    EXACT 
   9  <456>    |  5:    SUCCEED
                              could match...
   9  <456>    |  7:  OPEN1
   9  <456>    |  9:  PLUS
                           DIGIT can match 3 times out of 32767...
  Setting an EVAL scope, savestack=3
  12  <>    | 11:    CLOSE1
  12  <>    | 13:    END
Match successful!
456
Freeing REx: `(?<=f)(\d+)'