To understand the opcodes, you can pick up a copy of Advanced Perl Programming (the panther book) and read chapter 20. If you want to generate opcode listing of a Perl program, you'll need to recompile it with the <nobr>-DDEBUGGING</nobr> option. Then, to dump the syntax tree (with the opcodes), you'd do something like the following:

perl -Dx somescript.pl
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On japhy's posts, I have generally noticed that he is listing the regex opcodes. You can get these with the re pragma.

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

    'abc123def456' =~ /(?<=f)(\d+)/;
    print "$1\n";
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The above code will output how the regular expression compiled and will print out the exact steps the regex engine takes to match. Here's the output from 5.6.1 (ActiveState):

Compiling REx `(?<=f)(\d+)'
size 13 first at 1
synthetic stclass `ANYOF[0-9]'.
   1: IFMATCH[-1](7)
   3:   EXACT <f>(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 <abc> <123def456>    |  1:  IFMATCH[-1]
   2 <ab> <c123def456>    |  3:    EXACT <f>
                              failed...
                            failed...
  Setting an EVAL scope, savestack=3
   4 <abc1> <23def456>    |  1:  IFMATCH[-1]
   3 <abc> <123def456>    |  3:    EXACT <f>
                              failed...
                            failed...
  Setting an EVAL scope, savestack=3
   5 <abc12> <3def456>    |  1:  IFMATCH[-1]
   4 <abc1> <23def456>    |  3:    EXACT <f>
                              failed...
                            failed...
  Setting an EVAL scope, savestack=3
   9 <abc123def> <456>    |  1:  IFMATCH[-1]
   8 <abc123de> <f456>    |  3:    EXACT <f>
   9 <abc123def> <456>    |  5:    SUCCEED
                              could match...
   9 <abc123def> <456>    |  7:  OPEN1
   9 <abc123def> <456>    |  9:  PLUS
                           DIGIT can match 3 times out of 32767...
  Setting an EVAL scope, savestack=3
  12 <abc123def456> <>    | 11:    CLOSE1
  12 <abc123def456> <>    | 13:    END
Match successful!
456
Freeing REx: `(?<=f)(\d+)'
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The first part (lines numbered 1 to 13) are a breakdown of the regex.

Cheers,
Ovid

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