Regexp Java for password validation

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Try this:

^(?=.*[0-9])(?=.*[a-z])(?=.*[A-Z])(?=.*[@#$%^&+=])(?=\S+$).{8,}$

Explanation:

^                 # start-of-string
(?=.*[0-9])       # a digit must occur at least once
(?=.*[a-z])       # a lower case letter must occur at least once
(?=.*[A-Z])       # an upper case letter must occur at least once
(?=.*[@#$%^&+=])  # a special character must occur at least once
(?=\S+$)          # no whitespace allowed in the entire string
.{8,}             # anything, at least eight places though
$                 # end-of-string

It’s easy to add, modify or remove individual rules, since every rule is an independent “module”.

The (?=.*[xyz]) construct eats the entire string (.*) and backtracks to the first occurrence where [xyz] can match. It succeeds if [xyz] is found, it fails otherwise.

The alternative would be using a reluctant qualifier: (?=.*?[xyz]). For a password check, this will hardly make any difference, for much longer strings it could be the more efficient variant.

The most efficient variant (but hardest to read and maintain, therefore the most error-prone) would be (?=[^xyz]*[xyz]), of course. For a regex of this length and for this purpose, I would dis-recommend doing it that way, as it has no real benefits.

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