hbc/qualified_token: simplify token parsing

- Stop supporting archaic "user-repo/token" syntax
- Move regex for parsing tapped Cask token to tap_constants

Library/Homebrew/cask/lib/hbc/qualified_token.rb

 module Hbc
   module QualifiedToken
-    REPO_PREFIX = "homebrew-".freeze
-
-    # per https://github.com/Homebrew/homebrew/blob/4c7bc9ec3bca729c898ee347b6135ba692ee0274/Library/Homebrew/cmd/tap.rb#L121
-    USER_REGEX = /[a-z0-9_\-]+/
-
-    # per https://github.com/Homebrew/homebrew/blob/4c7bc9ec3bca729c898ee347b6135ba692ee0274/Library/Homebrew/cmd/tap.rb#L121
-    REPO_REGEX = /(?:#{REPO_PREFIX})?\w+/
-
-    # per https://github.com/caskroom/homebrew-cask/blob/master/CONTRIBUTING.md#generating-a-token-for-the-cask
-    TOKEN_REGEX = /[a-z0-9\-]+/
-
-    TAP_REGEX = %r{#{USER_REGEX}[/\-]#{REPO_REGEX}}
-
-    QUALIFIED_TOKEN_REGEX = %r{#{TAP_REGEX}/#{TOKEN_REGEX}}
-
     def self.parse(arg)
-      return nil unless arg.is_a?(String) && arg.downcase =~ /^#{QUALIFIED_TOKEN_REGEX}$/
-      path_elements = arg.downcase.split("/")
-      if path_elements.count == 2
-        # eg phinze-cask/google-chrome.
-        # Not certain this form is needed, but it was supported in the past.
-        token = path_elements[1]
-        dash_elements = path_elements[0].split("-")
-        repo = dash_elements.pop
-        dash_elements.pop if dash_elements.count > 1 && dash_elements[-1] + "-" == REPO_PREFIX
-        user = dash_elements.join("-")
-      else
-        # eg caskroom/cask/google-chrome
-        # per https://github.com/Homebrew/brew/blob/master/docs/brew-tap.md
-        user, repo, token = path_elements
-      end
-      repo.sub!(/^#{REPO_PREFIX}/, "")
+      return nil unless arg.is_a?(String)
+      return nil unless arg.downcase =~ HOMEBREW_TAP_CASK_REGEX
+      # eg caskroom/cask/google-chrome
+      # per https://github.com/Homebrew/brew/blob/master/docs/brew-tap.md
+      user, repo, token = arg.downcase.split("/")
       odebug "[user, repo, token] might be [#{user}, #{repo}, #{token}]"
       [user, repo, token]
     end

Library/Homebrew/tap_constants.rb

 # match taps' formulae, e.g. someuser/sometap/someformula
 HOMEBREW_TAP_FORMULA_REGEX = %r{^([\w-]+)/([\w-]+)/([\w+-.@]+)$}
+# match taps' casks, e.g. someuser/sometap/somecask
+HOMEBREW_TAP_CASK_REGEX = %r{^([\w-]+)/([\w-]+)/([a-z0-9\-]+)$}
 # match taps' directory paths, e.g. HOMEBREW_LIBRARY/Taps/someuser/sometap
 HOMEBREW_TAP_DIR_REGEX = %r{#{Regexp.escape(HOMEBREW_LIBRARY.to_s)}/Taps/([\w-]+)/([\w-]+)}
 # match taps' formula paths, e.g. HOMEBREW_LIBRARY/Taps/someuser/sometap/someformula