Using ?=. in regular expression

Question

I saw the phrase

^(?=.*[A-Z])(?=.*[a-z])(?=.*[0-9])[A-Za-z0-9_#@%\\*\\-]{8,24}$

in regex, which was password checking mechanism. I read few cou

Answer 1

(?=regex_here) is a positive lookahead. It is a zero-width assertion, meaning that it matches a location that is followed by the regex contained within (?= and ). To quote from the linked page:

lookaround actually matches characters, but then gives up the match, returning only the result: match or no match. That is why they are called "assertions". They do not consume characters in the string, but only assert whether a match is possible or not. Lookaround allows you to create regular expressions that are impossible to create without them, or that would get very longwinded without them.

The . is not part of the lookahead, because it matches any single character that is not a line terminator.