How to construct a regex expression for username validation? [closed] - php

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I'm working on constructing a PHP regex expression for a username validation that has the following constraints:
-must be 10-16 characters long, with a combo of alphabetic, numeric, and atleast one special character (*&^-_$)
-can't start with a numeric or special character
CORRECTION: the last SIX digits must be a month/date birthday (MMYYYY format). In order to further validate the username, the month/date must show the username is over 18 - if not, the username will not validate. Thank you in advance for any assistance! I've been stuck on this for a while.

Answer 1

Solution
You can do this with the following regex:
/(?=^.{10,16}$)(?=.+?[*&^_$-])[a-z].+?[01]\d{3}$/i
Here's a demo with some unit tests.
Explanation
/ delimiter
(?=^.{10,16}$) ensures there are 10-16 characters, start to finish
(?= starts a lookahead group
^ start of the string
.{10,16} ten to sixteen characters
$ end of the string
) ends the lookahead group
(?=.+?[*&^_$-]) ensures there is at least one special character in the set *&^_$-, and it's not first
(?= starts a lookahead group
.+? one or more characters, non-greedy
[*&^_$-] any character in the set *&^_$- (note the order; you must put - first or last, or escape it as \-)
) ends the lookahead group
[a-z] start with a letter
.+? match any characters in a non-greedy fashion, giving back as needed
[01]\d{3} match a 0 or 1 then 3 more digits
$ match the end of the string
/ closing delimiter
i make the match case-insensitive
Some Notes on Regex Construction
Note that there are multiple valid ways to do this. For pure efficiency, the solution above could be simplified somewhat to cut out a few steps for the processor.
But for readability, I like to go with something like the above. It's clear what each block, character set, or group does, which makes it readable and maintainable.
Something like /^[a-z](?=.*?[*&^_$-])[a-z0-9*&^_$-]{5,11}[01]\d{3}$ is hard to read and understand. What if you want to allow a 17 character username? You have to do a bunch of math to determine that you should change {5,11} to {5,12}. Or if you decide to allow the character #, you'd have to add it in two places (which, by the way, means that the regex already violates the DRY principle).
Bonus: Why Your Attempt Failed
You said in a comment that you tried this:
(?=^.{10,16}$)^[a-z][\d]*[_$^&*]?[a-z0-9]+
The first part, (?=^.{10,16}$), is fine. So is ^[a-z].
But [\d]* only matches zero or more digits; it wouldn't match a letter or special character. So, for example, a&a... would fail.
And [_$^&*]? only matches zero or one special characters. It would allow a username with no special characters to pass, but would fail one with 2 special characters.
[a-z0-9]+ only matches those characters, and you omit your last-four-characters-must-be-digits requirement.
You might find the explanation on regex101.com of your regex helpful. (Note: I have no affiliation with that site.)

Answer 2

You can use this regex :
^[a-zA-Z](?=.*[*&^_$-])[\w*&^$-]{5,11}[01]\d{3}$
Regex Breakup:
^ # Line start
[a-zA-Z] # # match an alhpabet
(?=.*[*&^_$-]) # lookahead to ensure there is at least one special char
[\w*&^$-]{5,11} # match 5 to 11 of allowed chars
[01]\d{3} # match digits 0/1 followed by 3 digits
$ # Line end
I used quantifier {5,11} because one char is matches at start and 4 are being matched in the end thus taking out 5 positions from desired {10,16} lengths.