module StringGsub
  def sub(pattern, replace=nil, &block)
    return super if pattern.is_a? String

    out = ""
    s = self
    x = pattern.match(s)
    if x
      out << x.pre_match
      if replace
        out << replace
      else
        block.call(x.match(0))
      end
      s = x.post_match
    end
    out << s
    out
  end

  def gsub(pattern, replace=nil, &block)
    return super if pattern.is_a? String

    out = ""
    s = self
    while (x = pattern.match(s))
      out << x.pre_match
      if replace
        out << replace
      else
        out << block.call(x[0])
      end
      s = x.post_match
    end
    out << s
    out
  end
end

class String
  prepend StringGsub
end