if g.sign == 0 {
		return
	}
	return p.appendGroup(r, g), s[2:]
}

// parseNamedClass parses a leading POSIX named character class like [:alnum:]
// from the beginning of s. If one is present, it appends the characters to r
// and returns the new slice r and the remainder of the string.
func (p *parser) parseNamedClass(s string, r []rune) (out []rune, rest string, err error) {
	if len(s) < 2 || s[0] != '[' || s[1] != ':' {