You're missing the whole point here. Square brackets are a character class. If I try to match on [a-z0-9], I'm specifying one character that falls in the class of characters from a-z and 0-9. That is, I'm trying to match one character that could be any of those in the class.
But if I try to match on [.], I'm specifying one character that falls in the class of characters that are a period. In other words, [a-z] could match 'a', or 'b', or 'c', etc., but [.] can only ever match '.'. So [.] is exactly equal to '.' Thus it's a useless use of a character class.
To use another example of yours, [\d\s] will match one character that is either a digit or a space character. It could match 9, or 8, or ' '. \d and \s retain their "magic" even in a character class. [\d] = \d = [0-9]
The lesson here is don't use single-character classes.