# = uri/common.rb
#
# Author:: Akira Yamada <akira@ruby-lang.org>
# Revision:: $Id: common.rb 22760 2009-03-04 09:21:12Z yugui $
# License:: 
#   You can redistribute it and/or modify it under the same term as Ruby.
#

module URI
  module REGEXP
    #
    # Patterns used to parse URI's
    #
    module PATTERN
      # :stopdoc:

      # RFC 2396 (URI Generic Syntax)
      # RFC 2732 (IPv6 Literal Addresses in URL's)
      # RFC 2373 (IPv6 Addressing Architecture)

      # alpha         = lowalpha | upalpha
      ALPHA = "a-zA-Z"
      # alphanum      = alpha | digit
      ALNUM = "#{ALPHA}\\d"

      # hex           = digit | "A" | "B" | "C" | "D" | "E" | "F" |
      #                         "a" | "b" | "c" | "d" | "e" | "f"
      HEX     = "a-fA-F\\d"
      # escaped       = "%" hex hex
      ESCAPED = "%[#{HEX}]{2}"
      # mark          = "-" | "_" | "." | "!" | "~" | "*" | "'" |
      #                 "(" | ")"
      # unreserved    = alphanum | mark
      UNRESERVED = "-_.!~*'()#{ALNUM}"
      # reserved      = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+" |
      #                 "$" | ","
      # reserved      = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+" | 
      #                 "$" | "," | "[" | "]" (RFC 2732)
      RESERVED = ";/?:@&=+$,\\[\\]"

      # domainlabel   = alphanum | alphanum *( alphanum | "-" ) alphanum
      DOMLABEL = "(?:[#{ALNUM}](?:[-#{ALNUM}]*[#{ALNUM}])?)"
      # toplabel      = alpha | alpha *( alphanum | "-" ) alphanum
      TOPLABEL = "(?:[#{ALPHA}](?:[-#{ALNUM}]*[#{ALNUM}])?)"
      # hostname      = *( domainlabel "." ) toplabel [ "." ]
      HOSTNAME = "(?:#{DOMLABEL}\\.)*#{TOPLABEL}\\.?"

      # :startdoc:
    end # PATTERN

    # :startdoc:
  end # REGEXP

  class Parser
    include REGEXP

    #
    # == Synopsis
    #
    #   URI::Parser.new([opts])
    #
    # == Args
    #
    # The constructor accepts a hash as options for parser.
    # Keys of options are pattern names of URI components
    # and values of options are pattern strings.
    # The constructor generetes set of regexps for parsing URIs.
    #
    # You can use the following keys:
    #
    #   * <tt>:ESCAPED</tt> (URI::PATTERN::ESCAPED in default)
    #   * <tt>:UNRESERVED</tt> (URI::PATTERN::UNRESERVED in default)
    #   * <tt>:DOMLABEL</tt> (URI::PATTERN::DOMLABEL in default)
    #   * <tt>:TOPLABEL</tt> (URI::PATTERN::TOPLABEL in default)
    #   * <tt>:HOSTNAME</tt> (URI::PATTERN::HOSTNAME in default)
    #
    # == Examples
    #
    #   p = URI::Parser.new(:ESCPAED => "(?:%[a-fA-F0-9]{2}|%u[a-fA-F0-9]{4})"
    #   u = p.parse("http://example.jp/%uABCD") #=> #<URI::HTTP:0xb78cf4f8 URL:http://example.jp/%uABCD>
    #   URI.parse(u.to_s) #=> raises URI::InvalidURIError
    #
    #   s = "http://examle.com/ABCD"
    #   u1 = p.parse(s) #=> #<URI::HTTP:0xb78c3220 URL:http://example.com/ABCD>
    #   u2 = URI.parse(s) #=> #<URI::HTTP:0xb78b6d54 URL:http://example.com/ABCD>
    #   u1 == u2 #=> true
    #   u1.eql?(u2) #=> false
    #
    def initialize(opts = {})
      @pattern = initialize_pattern(opts)
      @pattern.each_value {|v| v.freeze}
      @pattern.freeze

      @regexp = initialize_regexp(@pattern)
      @regexp.each_value {|v| v.freeze}
      @regexp.freeze
    end
    attr_reader :pattern, :regexp

    def split(uri)
      case uri
      when ''
	# null uri

      when @regexp[:ABS_URI]
	scheme, opaque, userinfo, host, port, 
	  registry, path, query, fragment = $~[1..-1]

	# URI-reference = [ absoluteURI | relativeURI ] [ "#" fragment ]

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

	# abs_path      = "/"  path_segments
	# net_path      = "//" authority [ abs_path ]

	# authority     = server | reg_name
	# server        = [ [ userinfo "@" ] hostport ]

	if !scheme
	  raise InvalidURIError, 
	    "bad URI(absolute but no scheme): #{uri}"
	end
	if !opaque && (!path && (!host && !registry))
	  raise InvalidURIError,
	    "bad URI(absolute but no path): #{uri}" 
	end

      when @regexp[:REL_URI]
	scheme = nil
	opaque = nil

	userinfo, host, port, registry, 
	  rel_segment, abs_path, query, fragment = $~[1..-1]
	if rel_segment && abs_path
	  path = rel_segment + abs_path
	elsif rel_segment
	  path = rel_segment
	elsif abs_path
	  path = abs_path
	end

	# URI-reference = [ absoluteURI | relativeURI ] [ "#" fragment ]

	# relativeURI   = ( net_path | abs_path | rel_path ) [ "?" query ]

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

	# authority     = server | reg_name
	# server        = [ [ userinfo "@" ] hostport ]

      else
	raise InvalidURIError, "bad URI(is not URI?): #{uri}"
      end

      path = '' if !path && !opaque # (see RFC2396 Section 5.2)
      ret = [
	scheme, 
	userinfo, host, port,         # X
	registry,                     # X
	path,                         # Y
	opaque,                       # Y
	query,
	fragment
      ]
      return ret
    end

    def parse(uri)
      scheme, userinfo, host, port, 
       	registry, path, opaque, query, fragment = self.split(uri)

      if scheme && URI.scheme_list.include?(scheme.upcase)
	URI.scheme_list[scheme.upcase].new(scheme, userinfo, host, port, 
                                           registry, path, opaque, query, 
                                           fragment, self)
      else
	Generic.new(scheme, userinfo, host, port, 
	   	    registry, path, opaque, query, 
	    	    fragment, self)
      end
    end

    def join(*str)
      u = self.parse(str[0])
      str[1 .. -1].each do |x|
	u = u.merge(x)
      end
      u
    end

    def extract(str, schemes = nil, &block)
      if block_given?
       	str.scan(make_regexp(schemes)) { yield $& }
	nil
      else
	result = []
	str.scan(make_regexp(schemes)) { result.push $& }
	result
      end
    end

    def make_regexp(schemes = nil)
      unless schemes
       	@regexp[:ABS_URI_REF]
      else
	/(?=#{Regexp.union(*schemes)}:)#{@pattern[:X_ABS_URI]}/x
      end
    end

    def escape(str, unsafe = @regexp[:UNSAFE])
      unless unsafe.kind_of?(Regexp)
        # perhaps unsafe is String object
        unsafe = Regexp.new("[#{Regexp.quote(unsafe)}]", false)
      end
      str.gsub(unsafe) do
        us = $&
        tmp = ''
        us.each_byte do |uc|
          tmp << sprintf('%%%02X', uc)
        end
        tmp
      end.force_encoding(Encoding::US_ASCII)
    end

    def unescape(str, escaped = @regexp[:ESCAPED])
      str.gsub(escaped) { [$&[1, 2].hex].pack('C') }.force_encoding(str.encoding)
    end

    @@to_s = Kernel.instance_method(:to_s)
    def inspect
      @@to_s.bind(self).call
    end

    private

    def initialize_pattern(opts = {})
      ret = {}
      ret[:ESCAPED] = escaped = (opts.delete(:ESCAPED) || PATTERN::ESCAPED)
      ret[:UNRESERVED] = unreserved = opts.delete(:UNRESERVED) || PATTERN::UNRESERVED
      ret[:RESERVED] = reserved = opts.delete(:RESERVED) || PATTERN::RESERVED
      ret[:DOMLABEL] = domlabel = opts.delete(:DOMLABEL) || PATTERN::DOMLABEL
      ret[:TOPLABEL] = toplabel = opts.delete(:TOPLABEL) || PATTERN::TOPLABEL
      ret[:HOSTNAME] = hostname = opts.delete(:HOSTNAME)

      # RFC 2396 (URI Generic Syntax)
      # RFC 2732 (IPv6 Literal Addresses in URL's)
      # RFC 2373 (IPv6 Addressing Architecture)

      # uric          = reserved | unreserved | escaped
      ret[:URIC] = uric = "(?:[#{unreserved}#{reserved}]|#{escaped})"
      # uric_no_slash = unreserved | escaped | ";" | "?" | ":" | "@" |
      #                 "&" | "=" | "+" | "$" | ","
      ret[:URIC_NO_SLASH] = uric_no_slash = "(?:[#{unreserved};?:@&=+$,]|#{escaped})"
      # query         = *uric
      ret[:QUERY] = query = "#{uric}*"
      # fragment      = *uric
      ret[:FRAGMENT] = fragment = "#{uric}*"

      # hostname      = *( domainlabel "." ) toplabel [ "." ]
      unless hostname
	ret[:HOSTNAME] = hostname = "(?:#{domlabel}\\.)*#{toplabel}\\.?"
      end

      # RFC 2373, APPENDIX B:
      # IPv6address = hexpart [ ":" IPv4address ]
      # IPv4address   = 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT
      # hexpart = hexseq | hexseq "::" [ hexseq ] | "::" [ hexseq ]
      # hexseq  = hex4 *( ":" hex4)
      # hex4    = 1*4HEXDIG
      #
      # XXX: This definition has a flaw. "::" + IPv4address must be
      # allowed too.  Here is a replacement.
      #
      # IPv4address = 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT
      ret[:IPV4ADDR] = ipv4addr = "\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}"
      # hex4     = 1*4HEXDIG
      hex4 = "[#{PATTERN::HEX}]{1,4}"
      # lastpart = hex4 | IPv4address
      lastpart = "(?:#{hex4}|#{ipv4addr})"
      # hexseq1  = *( hex4 ":" ) hex4
      hexseq1 = "(?:#{hex4}:)*#{hex4}"
      # hexseq2  = *( hex4 ":" ) lastpart
      hexseq2 = "(?:#{hex4}:)*#{lastpart}"
      # IPv6address = hexseq2 | [ hexseq1 ] "::" [ hexseq2 ]
      ret[:IPV6ADDR] = ipv6addr = "(?:#{hexseq2}|(?:#{hexseq1})?::(?:#{hexseq2})?)"

      # IPv6prefix  = ( hexseq1 | [ hexseq1 ] "::" [ hexseq1 ] ) "/" 1*2DIGIT
      # unused

      # ipv6reference = "[" IPv6address "]" (RFC 2732)
      ret[:IPV6REF] = ipv6ref = "\\[#{ipv6addr}\\]"

      # host          = hostname | IPv4address
      # host          = hostname | IPv4address | IPv6reference (RFC 2732)
      ret[:HOST] = host = "(?:#{hostname}|#{ipv4addr}|#{ipv6ref})"
      # port          = *digit
      port = '\d*'
      # hostport      = host [ ":" port ]
      ret[:HOSTPORT] = hostport = "#{host}(?::#{port})?"

      # userinfo      = *( unreserved | escaped |
      #                    ";" | ":" | "&" | "=" | "+" | "$" | "," )
      ret[:USERINFO] = userinfo = "(?:[#{unreserved};:&=+$,]|#{escaped})*"

      # pchar         = unreserved | escaped |
      #                 ":" | "@" | "&" | "=" | "+" | "$" | ","
      pchar = "(?:[#{unreserved}:@&=+$,]|#{escaped})"
      # param         = *pchar
      param = "#{pchar}*"
      # segment       = *pchar *( ";" param )
      segment = "#{pchar}*(?:;#{param})*"
      # path_segments = segment *( "/" segment )
      ret[:PATH_SEGMENTS] = path_segments = "#{segment}(?:/#{segment})*"

      # server        = [ [ userinfo "@" ] hostport ]
      server = "(?:#{userinfo}@)?#{hostport}"
      # reg_name      = 1*( unreserved | escaped | "$" | "," |
      #                     ";" | ":" | "@" | "&" | "=" | "+" )
      ret[:REG_NAME] = reg_name = "(?:[#{unreserved}$,;:@&=+]|#{escaped})+"
      # authority     = server | reg_name
      authority = "(?:#{server}|#{reg_name})"

      # rel_segment   = 1*( unreserved | escaped |
      #                     ";" | "@" | "&" | "=" | "+" | "$" | "," )
      ret[:REL_SEGMENT] = rel_segment = "(?:[#{unreserved};@&=+$,]|#{escaped})+"

      # scheme        = alpha *( alpha | digit | "+" | "-" | "." )
      ret[:SCHEME] = scheme = "[#{PATTERN::ALPHA}][-+.#{PATTERN::ALPHA}\\d]*"

      # abs_path      = "/"  path_segments
      ret[:ABS_PATH] = abs_path = "/#{path_segments}"
      # rel_path      = rel_segment [ abs_path ]
      ret[:REL_PATH] = rel_path = "#{rel_segment}(?:#{abs_path})?"
      # net_path      = "//" authority [ abs_path ]
      ret[:NET_PATH] = net_path = "//#{authority}(?:#{abs_path})?"

      # hier_part     = ( net_path | abs_path ) [ "?" query ]
      ret[:HIER_PART] = hier_part = "(?:#{net_path}|#{abs_path})(?:\\?(?:#{query}))?"
      # opaque_part   = uric_no_slash *uric
      ret[:OPAQUE_PART] = opaque_part = "#{uric_no_slash}#{uric}*"

      # absoluteURI   = scheme ":" ( hier_part | opaque_part )
      ret[:ABS_URI] = abs_uri = "#{scheme}:(?:#{hier_part}|#{opaque_part})"
      # relativeURI   = ( net_path | abs_path | rel_path ) [ "?" query ]
      ret[:REL_URI] = rel_uri = "(?:#{net_path}|#{abs_path}|#{rel_path})(?:\\?#{query})?"

      # URI-reference = [ absoluteURI | relativeURI ] [ "#" fragment ]
      ret[:URI_REF] = uri_ref = "(?:#{abs_uri}|#{rel_uri})?(?:##{fragment})?"

      ret[:X_ABS_URI] = "
        (#{scheme}):                           (?# 1: scheme)
        (?:
           (#{opaque_part})                    (?# 2: opaque)
        |
           (?:(?:
             //(?:
                 (?:(?:(#{userinfo})@)?        (?# 3: userinfo)
                   (?:(#{host})(?::(\\d*))?))? (?# 4: host, 5: port)
               |
                 (#{reg_name})                 (?# 6: registry)
               )
             |
             (?!//))                           (?# XXX: '//' is the mark for hostport)
             (#{abs_path})?                    (?# 7: path)
           )(?:\\?(#{query}))?                 (?# 8: query)
        )
        (?:\\#(#{fragment}))?                  (?# 9: fragment)
      "

      ret[:X_REL_URI] = "
        (?:
          (?:
            //
            (?:
              (?:(#{userinfo})@)?       (?# 1: userinfo)
                (#{host})?(?::(\\d*))?  (?# 2: host, 3: port)
            |
              (#{reg_name})             (?# 4: registry)
            )
          )
        |
          (#{rel_segment})              (?# 5: rel_segment)
        )?
        (#{abs_path})?                  (?# 6: abs_path)
        (?:\\?(#{query}))?              (?# 7: query)
        (?:\\#(#{fragment}))?           (?# 8: fragment)
      "

      ret
    end

    def initialize_regexp(pattern)
      ret = {}

      # for URI::split
      ret[:ABS_URI] = Regexp.new('^' + pattern[:X_ABS_URI] + '$', Regexp::EXTENDED)
      ret[:REL_URI] = Regexp.new('^' + pattern[:X_REL_URI] + '$', Regexp::EXTENDED)

      # for URI::extract
      ret[:URI_REF]     = Regexp.new(pattern[:URI_REF])
      ret[:ABS_URI_REF] = Regexp.new(pattern[:X_ABS_URI], Regexp::EXTENDED)
      ret[:REL_URI_REF] = Regexp.new(pattern[:X_REL_URI], Regexp::EXTENDED)

      # for URI::escape/unescape
      ret[:ESCAPED] = Regexp.new(pattern[:ESCAPED])
      ret[:UNSAFE]  = Regexp.new("[^#{pattern[:UNRESERVED]}#{pattern[:RESERVED]}]")

      # for Generic#initialize
      ret[:SCHEME]   = Regexp.new("^#{pattern[:SCHEME]}$")
      ret[:USERINFO] = Regexp.new("^#{pattern[:USERINFO]}$")
      ret[:HOST]     = Regexp.new("^#{pattern[:HOST]}$")
      ret[:PORT]     = Regexp.new("^#{pattern[:PORT]}$")
      ret[:OPAQUE]   = Regexp.new("^#{pattern[:OPAQUE_PART]}$")
      ret[:REGISTRY] = Regexp.new("^#{pattern[:REG_NAME]}$")
      ret[:ABS_PATH] = Regexp.new("^#{pattern[:ABS_PATH]}$")
      ret[:REL_PATH] = Regexp.new("^#{pattern[:REL_PATH]}$")
      ret[:QUERY]    = Regexp.new("^#{pattern[:QUERY]}$")
      ret[:FRAGMENT] = Regexp.new("^#{pattern[:FRAGMENT]}$")

      ret
    end
  end # class Parser

  DEFAULT_PARSER = Parser.new
  DEFAULT_PARSER.pattern.each_pair do |sym, str|
    unless REGEXP::PATTERN.const_defined?(sym)
      REGEXP::PATTERN.const_set(sym, str)
    end
  end
  DEFAULT_PARSER.regexp.each_pair do |sym, str|
    const_set(sym, str)
  end

  module Util # :nodoc:
    def make_components_hash(klass, array_hash)
      tmp = {}
      if array_hash.kind_of?(Array) &&
          array_hash.size == klass.component.size - 1
        klass.component[1..-1].each_index do |i|
          begin
            tmp[klass.component[i + 1]] = array_hash[i].clone
          rescue TypeError
            tmp[klass.component[i + 1]] = array_hash[i]
          end
        end

      elsif array_hash.kind_of?(Hash)
        array_hash.each do |key, value|
          begin
            tmp[key] = value.clone
          rescue TypeError
            tmp[key] = value
          end
        end
      else
        raise ArgumentError, 
          "expected Array of or Hash of components of #{klass.to_s} (#{klass.component[1..-1].join(', ')})"
      end
      tmp[:scheme] = klass.to_s.sub(/\A.*::/, '').downcase

      return tmp
    end
    module_function :make_components_hash
  end

  module Escape
    #
    # == Synopsis
    #
    #   URI.escape(str [, unsafe])
    #
    # == Args
    #
    # +str+::
    #   String to replaces in.
    # +unsafe+::
    #   Regexp that matches all symbols that must be replaced with codes.
    #   By default uses <tt>REGEXP::UNSAFE</tt>.
    #   When this argument is a String, it represents a character set.
    #
    # == Description
    #
    # Escapes the string, replacing all unsafe characters with codes.
    #
    # == Usage
    #
    #   require 'uri'
    #
    #   enc_uri = URI.escape("http://example.com/?a=\11\15")
    #   p enc_uri
    #   # => "http://example.com/?a=%09%0D"
    #
    #   p URI.unescape(enc_uri)
    #   # => "http://example.com/?a=\t\r"
    #
    #   p URI.escape("@?@!", "!?")
    #   # => "@%3F@%21"
    #
    def escape(*arg)
      DEFAULT_PARSER.escape(*arg)
    end
    alias encode escape
    #
    # == Synopsis
    #
    #   URI.unescape(str)
    #
    # == Args
    #
    # +str+::
    #   Unescapes the string.
    #
    # == Usage
    #
    #   require 'uri'
    #
    #   enc_uri = URI.escape("http://example.com/?a=\11\15")
    #   p enc_uri
    #   # => "http://example.com/?a=%09%0D"
    #
    #   p URI.unescape(enc_uri)
    #   # => "http://example.com/?a=\t\r"
    #
    def unescape(*arg)
      DEFAULT_PARSER.unescape(*arg)
    end
    alias decode unescape
  end

  extend Escape
  include REGEXP

  @@schemes = {}
  def self.scheme_list
    @@schemes
  end
  
  #
  # Base class for all URI exceptions.
  #
  class Error < StandardError; end
  #
  # Not a URI.
  #
  class InvalidURIError < Error; end
  #
  # Not a URI component.
  #
  class InvalidComponentError < Error; end
  #
  # URI is valid, bad usage is not.
  #
  class BadURIError < Error; end

  #
  # == Synopsis
  #
  #   URI::split(uri)
  #
  # == Args
  #
  # +uri+::
  #   String with URI.
  #
  # == Description
  #
  # Splits the string on following parts and returns array with result:
  #
  #   * Scheme
  #   * Userinfo
  #   * Host
  #   * Port
  #   * Registry
  #   * Path
  #   * Opaque
  #   * Query
  #   * Fragment
  # 
  # == Usage
  #
  #   require 'uri'
  #
  #   p URI.split("http://www.ruby-lang.org/")
  #   # => ["http", nil, "www.ruby-lang.org", nil, nil, "/", nil, nil, nil]
  #
  def self.split(uri)
    DEFAULT_PARSER.split(uri)
  end

  #
  # == Synopsis
  #
  #   URI::parse(uri_str)
  #
  # == Args
  #
  # +uri_str+::
  #   String with URI.
  #
  # == Description
  #
  # Creates one of the URI's subclasses instance from the string.
  #  
  # == Raises
  #
  # URI::InvalidURIError
  #   Raised if URI given is not a correct one.
  #
  # == Usage
  #
  #   require 'uri'
  #
  #   uri = URI.parse("http://www.ruby-lang.org/")
  #   p uri
  #   # => #<URI::HTTP:0x202281be URL:http://www.ruby-lang.org/>
  #   p uri.scheme 
  #   # => "http" 
  #   p uri.host 
  #   # => "www.ruby-lang.org" 
  # 
  def self.parse(uri)
    DEFAULT_PARSER.parse(uri)
  end

  #
  # == Synopsis
  #
  #   URI::join(str[, str, ...])
  #
  # == Args
  #
  # +str+::
  #   String(s) to work with
  #
  # == Description
  #
  # Joins URIs.
  #
  # == Usage
  #
  #   require 'uri'
  #
  #   p URI.join("http://localhost/","main.rbx")
  #   # => #<URI::HTTP:0x2022ac02 URL:http://localhost/main.rbx>
  #
  def self.join(*str)
    DEFAULT_PARSER.join(*str)
  end

  #
  # == Synopsis
  #
  #   URI::extract(str[, schemes][,&blk])
  #
  # == Args
  #
  # +str+:: 
  #   String to extract URIs from.
  # +schemes+::
  #   Limit URI matching to a specific schemes.
  #
  # == Description
  #
  # Extracts URIs from a string. If block given, iterates through all matched URIs.
  # Returns nil if block given or array with matches.
  #
  # == Usage
  #
  #   require "uri"
  #
  #   URI.extract("text here http://foo.example.org/bla and here mailto:test@example.com and here also.")
  #   # => ["http://foo.example.com/bla", "mailto:test@example.com"]
  #
  def self.extract(str, schemes = nil, &block)
    DEFAULT_PARSER.extract(str, schemes, &block)
  end

  #
  # == Synopsis
  #
  #   URI::regexp([match_schemes])
  #
  # == Args
  #
  # +match_schemes+:: 
  #   Array of schemes. If given, resulting regexp matches to URIs
  #   whose scheme is one of the match_schemes.
  # 
  # == Description
  # Returns a Regexp object which matches to URI-like strings.
  # The Regexp object returned by this method includes arbitrary
  # number of capture group (parentheses).  Never rely on it's number.
  # 
  # == Usage
  #
  #   require 'uri'
  #
  #   # extract first URI from html_string
  #   html_string.slice(URI.regexp)
  # 
  #   # remove ftp URIs
  #   html_string.sub(URI.regexp(['ftp'])
  # 
  #   # You should not rely on the number of parentheses
  #   html_string.scan(URI.regexp) do |*matches|
  #     p $&
  #   end
  #
  def self.regexp(schemes = nil)
    DEFAULT_PARSER.make_regexp(schemes)
  end

end

module Kernel
  # alias for URI.parse.
  #
  # This method is introduced at 1.8.2.
  def URI(uri_str) # :doc:
    URI.parse(uri_str)
  end
  module_function :URI
end