matcher.go
func (m *FixedLengthMatcher) Match(s string) (offset int, matchStr string) {
	if len(s) < m.length || !m.match(s) {
		offset = -1
		return
	}
	return m.length, s[:m.length]
}
func (m *FixedLengthMatcher) Match(s string) (offset int, matchStr string) {
	if len(s) < m.length || !m.match(s) {
		offset = -1
		return
	}
	return m.length, s[:m.length]
}
// Match against fixed length pattern.