Hold up now, that example hash doesn’t have a $ prefix!

Well for this there is the option for a hash type to set itself as a fall through if a matching hash doesn’t exist. This is good for legacy password types that don’t follow the convention.

func (p *plainPasswd) ApplyPasswd(passwd *passwd.Passwd) {
	passwd.Register("plain", p)
	passwd.SetFallthrough(p)
}

https://github.com/sour-is/go-passwd/blob/main/passwd_test.go#L28-L31