steveklabnik/abnf

require 'pp'
require 'abnf'

# URI-reference [RFC2396]
pp ABNF.regexp_tree(<<-'End')
      URI-reference = [ absoluteURI | relativeURI ] [ "#" fragment ]
      absoluteURI   = scheme ":" ( hier_part | opaque_part )
      relativeURI   = ( net_path | abs_path | rel_path ) [ "?" query ]

      hier_part     = ( net_path | abs_path ) [ "?" query ]
      opaque_part   = uric_no_slash *uric

      uric_no_slash = escaped | unreserved | ";" | "?" | ":" | "@" |
              "&" | "=" | "+" | "$" | ","

      net_path      = "//" authority [ abs_path ]
      abs_path      = "/"  path_segments
      rel_path      = rel_segment [ abs_path ]

      rel_segment   = 1*( escaped | unreserved |
              ";" | "@" | "&" | "=" | "+" | "$" | "," )

      scheme        = alpha *( alpha | digit | "+" | "-" | "." )

      authority     = server | reg_name

      reg_name      = 1*( escaped | unreserved | "$" | "," |
              ";" | ":" | "@" | "&" | "=" | "+" )

      server        = [ [ userinfo "@" ] hostport ]
      userinfo      = *( escaped | unreserved |
             ";" | ":" | "&" | "=" | "+" | "$" | "," )

      hostport      = host [ ":" port ]
      host          = hostname | IPv4address
      hostname      = *( domainlabel "." ) toplabel [ "." ]
      domainlabel   = alphanum | alphanum *( alphanum | "-" ) alphanum
      toplabel      = alpha | alpha *( alphanum | "-" ) alphanum
      IPv4address   = 1*digit "." 1*digit "." 1*digit "." 1*digit
      port          = *digit

      path          = [ abs_path | opaque_part ]
      path_segments = segment *( "/" segment )
      segment       = *pchar *( ";" param )
      param         = *pchar
      pchar         = escaped | unreserved |
              ":" | "@" | "&" | "=" | "+" | "$" | ","

      query         = *uric

      fragment      = *uric

      uric          = reserved | unreserved | escaped
      reserved      = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+" |
              "$" | ","
      unreserved    = alphanum | mark
      mark          = "-" | "_" | "." | "!" | "~" | "*" | "'" |
              "(" | ")"

      escaped       = "%" hex hex
      hex           = digit | "A" | "B" | "C" | "D" | "E" | "F" |
                  "a" | "b" | "c" | "d" | "e" | "f"

      alphanum      = alpha | digit
      alpha         = lowalpha | upalpha

      lowalpha = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" | "i" |
         "j" | "k" | "l" | "m" | "n" | "o" | "p" | "q" | "r" |
         "s" | "t" | "u" | "v" | "w" | "x" | "y" | "z"
      upalpha  = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" |
         "J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" |
         "S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z"
      digit    = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
         "8" | "9"
End