regexp dot with square brackets or not

What is the exact difference between .* and [.]*?

I have tried to put these inside parentheses, for back-reference, and saw the results are not the same although I don't understand why.

The . is supposed to match any single character.

So I guess whether it's inside square brackets or not should not be important with the * (match zero or more) operator. But it is. Why?

Solution 1:

In .*, a dot is a special character matching any character but a newline (it will match all characters if you specify a DOTALL modifier). In a character class ([.]), a dot loses its special meaning and starts matching a literal dot.

It is a universal behavior across all regex flavors.

Solution 2:

• . matches any character, apart from newline.
• \. only matches a literal ".".
• [.] is equivalent to [\.] or \. This is just for convenience - because you almost certainty don't want it to match "any character", in the context of a character group.

Bonus -- If you use my ruby gem, you can easily experiment with stuff like this:

/./.examples # => ["a", "b", "c", "d", "e"]
/\./.examples # => ["."]
/[.]/.examples # => ["."]