^[a-zA-Z0-9]*[a-zA-Z0-9 _][a-zA-Z0-9]{2,24}$
That's what I have right now.
I want to require an alphanumeric first; allow for alphanumerics, underscores, hyphens, periods, and spaces; require that it end with an alphanumeric. But I only want to allow as many as 3 of those special characters.
I'm primarily confused about how to limit the number of the special characters.
You can also use this regex:
/^(?!(?:[a-z\d]*[_. -]){4})[a-z\d][\w. -]{0,22}[a-z\d]$/i
The look-ahead (?!(?:[a-z\d]*[_. -]){4}) is to check that there are less than 4 appearances of invalid characters. If there are 4 or more, then the pattern inside the negative look-ahead would match, and make the look-ahead fail.
Since the string must start and end with alphanumeric, and the length is at least 2, it is possible to specify [a-z\d] as start and end of the string. The rest of the character in between can contain any of [a-zA-Z0-9_. -] repeated 0 to 22 times, since 2 characters are already use for the starting ending alphanumeric.
?: and ?= ?
/i flag. So it's necessary to drop it and manually specify uppercase letters as valid. This means that the pattern should be ^(?!(?:[a-zA-Z\d]*[_. -]){4})[a-zA-Z\d][\w. -]{2,22}[a-zA-Z\d]$ Thanks, nhahtdh.
(?!pattern), with a non-capturing group (?:pattern) repeating 4 times.Okay, this should be the last edit: Didn't think about the total character limit. Added look-ahead (assuming your flavor of regex supports it).
There may be a better way than this, but it's not coming to me (maybe using lookaheads). Here's what I can think of:
^(?=^.{2,24}$)[a-zA-Z0-9]+([a-zA-Z0-9]*[_\-. ]){0,3}[a-zA-Z0-9]+$
It's not too pretty but it should work.
^[a-zA-Z0-9]+(([a-zA-Z0-9]*[_\-. ]){0,3}[a-zA-Z0-9]+)?$, and the length check can be done separately.^[a-zA-Z0-9]+[ _.-]?[a-zA-Z0-9]*[ _.-]?[a-zA-Z0-9]*[ _.-]?[a-zA-Z0-9]+$
There's probably a better way to do this, but I think you could just have the invalid characters be optional, appearing at most three times, with the other valid characters showing up 0 or more times in between them
