code: public EndpointInfoBuilder regexPathPrefix(String regexPathPrefix) {
          requireNonNull(regexPathPrefix, "regexPathPrefix");
          checkState(fragment == null, "regexPathPrefix cannot be set with fragment: %s", fragment);
          this.regexPathPrefix = regexPathPrefix;
          return this;
      }
docs: Sets the prefix of the pathMapping if the {@code pathMapping} is a regular expression string and
  the prefix exists.