YAPE::Regex::Explain provides a good explanation: 

$ perl -E '
use YAPE::Regex::Explain;
my $e = YAPE::Regex::Explain->new(q{^-?\d+$})->explain;
say $e;
'
The regular expression:

(?-imsx:^-?\d+$)

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
----------------------------------------------------------------------
  -?                       '-' (optional (matching the most amount
                           possible))
----------------------------------------------------------------------
  \d+                      digits (0-9) (1 or more times (matching
                           the most amount possible))
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

[download]

I'd also recommend the following documentation:

-- Ken

In reply to Re^4: Die vs. Exit by kcott
in thread Die vs. Exit by shortyfw06

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