code: func (r *Route) HeadersRegexp(pairs ...string) *Route {
  	if r.err == nil {
  		var headers map[string]*regexp.Regexp
  		headers, r.err = mapFromPairsToRegex(pairs...)
  		return r.addMatcher(headerRegexMatcher(headers))
  	}
  	return r
  }
docs: // HeadersRegexp accepts a sequence of key/value pairs, where the value has regex
  // support. For example:
  //
  //     r := mux.NewRouter()
  //     r.HeadersRegexp("Content-Type", "application/(text|json)",
  //               "X-Requested-With", "XMLHttpRequest")
  //
  // The above route will only match if both the request header matches both regular expressions.
  // If the value is an empty string, it will match any value if the key is set.
  // Use the start and end of string anchors (^ and $) to match an exact value.