Provided you do read perlre and the other fine references so that you have a basis for understanding, you might find YAPE::Regex::Explain (by our own japhy) to be of use.

perl -MYAPE::Regex::Explain -e "print YAPE::Regex::Explain->new(shift)->explain;" "^\s*$"

yields

The regular expression:

(?-imsx:^\s*$)

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
----------------------------------------------------------------------
  \s*                      whitespace (\n, \r, \t, \f, and " ") (0 or
                           more times (matching the most amount
                           possible))
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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