code: func emailTemplateFilenameRegexp(c context.Context) *regexp.Regexp {
  	appId := info.AppID(c)
  	pattern := fmt.Sprintf(`^%s+/email-templates/([a-z][a-z0-9_]*)\.template$`, regexp.QuoteMeta(appId))
  	return regexp.MustCompile(pattern)
  }
docs: // emailTemplateFilenameRegexp returns a regular expression for email template
  // file names.