regex for URL matching

regex-url

From rfc3986:

   As the "first-match-wins" algorithm is identical to the "greedy"
   disambiguation method used by POSIX regular expressions, it is
   natural and commonplace to use a regular expression for parsing the
   potential five components of a URI reference.

   The following line is the regular expression for breaking-down a
   well-formed URI reference into its components.

      ^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?
       12            3  4          5       6  7        8 9

   The numbers in the second line above are only to assist readability;
   they indicate the reference points for each subexpression (i.e., each
   paired parenthesis).  We refer to the value matched for subexpression
   <n> as $<n>.  For example, matching the above expression to

      http://www.ics.uci.edu/pub/ietf/uri/#Related

   results in the following subexpression matches:

      $1 = http:
      $2 = http
      $3 = //www.ics.uci.edu
      $4 = www.ics.uci.edu
      $5 = /pub/ietf/uri/
      $6 = <undefined>
      $7 = <undefined>
      $8 = #Related
      $9 = Related

   where <undefined> indicates that the component is not present, as is
   the case for the query component in the above example.  Therefore, we
   can determine the value of the five components as

      scheme    = $2
      authority = $4
      path      = $5
      query     = $7
      fragment  = $9

   Going in the opposite direction, we can recreate a URI reference from
   its components by using the algorithm of Section 5.3.