Question

Can anyone tell me what “?=” does when using regex?
Here is an example of the code fragment I am trying to decipher:

password.matches("(?=.*\\d.*\\d.*)^[\\w]{8}.*$");

Thanks.

Answer 1

It’s a positive lookahead. In that particular expression, it is saying that your password must have at least two digits (\d).

Also note that a lookahead doesn’t consume input, it is merely an assertion.

For example, in your regex, the lookahead part ((?=.*\\d.*\\d.*)) asserts that your password contains at least two digits, and the rest of the expression consumes the entire string, and tries to match at least 8 word characters (i.e., [a-zA-Z_0-9]) at the beginning of the string.