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

##</code><code>##

perl -MYAPE::Regexp::Explain -e "print YAPE::Regex::Explain->new('\btest(?: more)\b$')->explain();"

##</code><code>##

The regular expression:

(?-imsx:\btest(?: more)\b$)

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):
----------------------------------------------------------------------
  \b                       the boundary between a word char (\w) and
                           something that is not a word char
----------------------------------------------------------------------
  test                     'test'
----------------------------------------------------------------------
  (?:                      group, but do not capture:
----------------------------------------------------------------------
     more                    ' more'
----------------------------------------------------------------------
  )                        end of grouping
----------------------------------------------------------------------
  \b                       the boundary between a word char (\w) and
                           something that is not a word char
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

##</code><code>##

#!/usr/bin/perl

use re 'debug';

m/
  \b
  test regexp
  \b
/x;