Using ?=. in regular expression - regex

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 courses about regular expressions, but I never saw combination ?=. explained.
I want know how it works. In the example it is searching for at least one capital letter, one small letter and one number. I guess it's something like "if".

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.

Answer 2

Although i am a newbie to regex but what i understand about the above regex is
1- ?= is positive lookahead i.e. it matches the expression by looking ahead and sees if there is any pattern that matches your search paramater like [A-Z]
2- .* makes sure that they can be 0 or more number of characters before your matching expression i.e. it makes sure that u can lookahead till the end of the input string to find a match.
In short * is a quantifier which says 0 or more so if:
For instance u changed * with ? for [A-Z] part then your expression will only return true if ur 1st or 2nd letter is capital. OR if u changed it with + then ur expression will return true if any letter other than the first is a capital letter

Answer 3

^ asserts position at start of the string
Positive Lookahead (?=\D*\d)
Assert that the Regex below matches
\D matches any character that's not a digit (equivalent to [^0-9])
matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
\d matches a digit (equivalent to [0-9])
Positive Lookahead (?=[^a-z]*[a-z])
Assert that the Regex below matches
Match a single character not present in the list below [^a-z]
matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
Match a single character present in the list below [a-z]
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
Positive Lookahead (?=[^A-Z]*[A-Z])
Assert that the Regex below matches
Match a single character not present in the list below [^A-Z]
matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
Match a single character present in the list below [A-Z]
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
. matches any character (except for line terminators)
{8,30} matches the previous token between 8 and 30 times, as many times as possible, giving back as needed (greedy)
$ asserts position at the end of the string, or before the line terminator right at the end of the string (if any)