I want an alphanumeric (not a word) string between 3 and 50 characters long. It can have no more that 3 whitespaces in it. The best I've come up with is: ^[a-zA-Z0-9]{3,50}[\s]{0,3}$ but this doesn't work.
Can someone please point me in the right direction?

CodePudding user response：

As Cary Swoveland has suggested in the comments, you may use a negative Lookahead to ensure that the string doesn't have more than 3 whitespace characters. The full pattern would be something like this:

^(?!(?:.*\h){4})[A-Za-z0-9\h]{3,50}$

Demo.

Note that I used \h instead of \s here in order to only match horizontal whitespace characters.

If you need to prevent whitespace characters from appearing at the beginning/end of the string, you could adjust the Lookahead and add a negative Lookbehind at the end:

^(?!\h|(?:.*\h){4})[A-Za-z0-9\h]{3,50}(?<!\h)$

Demo.