code: func (a *Assertions) Regexp(rx interface{}, str interface{}, msgAndArgs ...interface{}) bool {
  	if h, ok := a.t.(tHelper); ok {
  		h.Helper()
  	}
  	return Regexp(a.t, rx, str, msgAndArgs...)
  }
docs: // Regexp asserts that a specified regexp matches a string.
  //
  //  a.Regexp(regexp.MustCompile("start"), "it's starting")
  //  a.Regexp("start...$", "it's not starting")