#!/usr/bin/perl
use warnings;
use strict;

use YAPE::Regex::Explain;

my $REx = '/^list\s+(\w+)(?:,(\w+))*$/m';

my $exp = YAPE::Regex::Explain->new($REx)->explain;
print $exp;

__END__
The regular expression:

(?-imsx:/^list\s+(\w+)(?:,(\w+))*$/m)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  /                        '/'
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  list                     'list'
----------------------------------------------------------------------
  \s+                      whitespace (\n, \r, \t, \f, and " ") (1 or
                           more times (matching the most amount
                           possible))
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    \w+                      word characters (a-z, A-Z, 0-9, _) (1 or
                             more times (matching the most amount
                             possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  (?:                      group, but do not capture (0 or more times
                           (matching the most amount possible)):
----------------------------------------------------------------------
    ,                        ','
----------------------------------------------------------------------
    (                        group and capture to \2:
----------------------------------------------------------------------
      \w+                      word characters (a-z, A-Z, 0-9, _) (1
                               or more times (matching the most
                               amount possible))
----------------------------------------------------------------------
    )                        end of \2
----------------------------------------------------------------------
  )*                       end of grouping
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
  /m                       '/m'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

Process completed successfully