A handy tool for understanding complex regexes is YAPE::Regex::Explain

$ perl -e 'use YAPE::Regex::Explain;print YAPE::Regex::Explain->new("^
+(?:https?|ftp):")->explain;'
The regular expression:

(?-imsx:^(?:https?|ftp):)

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
----------------------------------------------------------------------
  (?:                      group, but do not capture:
----------------------------------------------------------------------
    http                     'http'
----------------------------------------------------------------------
    s?                       's' (optional (matching the most amount
                             possible))
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    ftp                      'ftp'
----------------------------------------------------------------------
  )                        end of grouping
----------------------------------------------------------------------
  :                        ':'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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flounder