How can you tell what ($|--$) is really trying to match? use re 'debug'!

#!/usr/local/bin/perl -w
use strict;
use re 'debug';
"xyz" =~ /arbITRary CharS ($|--$)/;
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When run (actually, when compiled), you get this output:

compiling RE `arbITRary CharS ($|--$)'
size 17 first at 1
   1: EXACT <arbITRary CharS >(7)
   7: OPEN1(9)
   9:   BRANCH(11)
  10:     EOL(15)
  11:   BRANCH(15)
  12:     EXACT <-->(14)
  14:     EOL(15)
  15: CLOSE1(17)
  17: END(0)
anchored `arbITRary CharS ' at 0 (checking anchored) minlen 16 

OPEN1(9) corresponds to the opening (capturing) bracket. It's followed by one of two BRANCHes: EOL (given by $) or EXACT <--> and an EOL (given by --$).

So Perl can tell you exactly what it thinks you mean by your regular expression.

use re 'debug' also prints out information about the match, but in this case we don't care about it...