code: func IsHTTPHeaderName(value string) []string {
  	if !httpHeaderNameRegexp.MatchString(value) {
  		return []string{RegexError(httpHeaderNameErrMsg, httpHeaderNameFmt, "X-Header-Name")}
  	}
  	return nil
  }
docs: // IsHTTPHeaderName checks that a string conforms to the Go HTTP library's
  // definition of a valid header field name (a stricter subset than RFC7230).