code: func (a *Assertions) Regexpf(rx interface{}, str interface{}, msg string, args ...interface{}) {
  	if h, ok := a.t.(tHelper); ok {
  		h.Helper()
  	}
  	Regexpf(a.t, rx, str, msg, args...)
  }
docs: // Regexpf asserts that a specified regexp matches a string.
  //
  //  a.Regexpf(regexp.MustCompile("start", "error message %s", "formatted"), "it's starting")
  //  a.Regexpf("start...$", "it's not starting", "error message %s", "formatted")