You may find japhy's YAPE::Regex::Explain module to be useful in answering these types of question. You'll need to install YAPE::Regex first. This is waht it had to say about your example regex:

#!/usr/bin/perl -w

use strict;
use YAPE::Regex::Explain;

my $exp = YAPE::Regex::Explain->new('^-?\.?\d+(?:\.\d+)?$');

print $exp->explain;


The regular expression:

(?-imsx:^-?\.?\d+(?:\.\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))
----------------------------------------------------------------------
  \.?                      '.' (optional (matching the most amount
                           possible))
----------------------------------------------------------------------
  \d+                      digits (0-9) (1 or more times (matching
                           the most amount possible))
----------------------------------------------------------------------
  (?:                      group, but do not capture (optional
                           (matching the most amount possible)):
----------------------------------------------------------------------
    \.                       '.'
----------------------------------------------------------------------
    \d+                      digits (0-9) (1 or more times (matching
                             the most amount possible))
----------------------------------------------------------------------
  )?                       end of grouping
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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Which may or may not help :-). Note that it acts as if the regex you supply had (?-imsx: ... ) wrapped around it, which could be a bit confusing.