route_regexp.rb

def build(str)
  str = str.dup
  str.prepend '\A'
  str.gsub! %r{/:([a-z]+)}, '/(?<\1>[^\/]+)'
  str << '(?<format>\.[a-z]\w*)?'
  str << '\z'
  Regexp.new(str)
end

rx = build('/posts/:id/comments')

m = rx.match('/posts/123/comments')
puts m[:id] # 123

m = rx.match('/posts/123/comments.html')
puts m[:id] # 123
puts m[:format] # .html