[draegtun]

Have a look at Rebol parse dialect for an alternative mind blowing way of doing things!

Here's is OP example ported to parse:

  ; build some sub rules
  r: context [
    domain: charset [#"a" - #"z" #"-"]
    proto:  ["http" | "https"]
    tld:    ["com" | "org"]
  ]
  
  ; simple URI rule
  rule: [
    copy proto r/proto
    "://"
    some [copy d some r/domain #"." (append domain d)]
    copy tld r/tld
    ["/" | end]
  ]
  
  ; capture
  proto: tld: d: none
  domain: []
  
  ; usage example
  parse "http://www.example.com/" rule

  ; Above returns true with following variables set to...
  ;
  ; proto  = "http"
  ; domain = ["www" "example"]
  ; tld    = "com"

Here's a nice tutorial on parse (though it's probably Rebol 2 centric) - http://www.codeconscious.com/rebol/parse-tutorial.html