Question

I’m learning about regular expressions and have one question; the answer will help me to understand regexes better.

The input is a username. This username should include at least 4 lower case chars (a-z), one upper char (A-Z) and 2 numbers. It should also have a maximum of 10 chars in total. How can I make a regular expression to test for these requirements?

Answer 1

Use a regex with lookaheads / lookbehinds for each condition. Something like below:

^(?=(.*[a-z]){4})(?=.*[A-Z])(?=(.*\d){2}).{7,10}$

I think the regex is self explanatory, tell me if you want me to explain on each part.

Ok, explanation as per OP’s request:

(?=ABC),(?!ABC) and (?<=ABC), (?<!ABC) – are lookaheads and lookbehinds that match groups before / after your expression and don’t include them in the results. The one with = are positive and one with ! are negative.

Here for example, (?=(.*[a-z]){4}) ensures that the main expression (.{7,10}) has at least 4 lower case characters. Similarly, we have one for each condition. .{7,10} ensure max 10 ( minimum 7 – 4 lower + 1 upper + 2 digits )

Having such tightly constrained passwords (whoa usernames like this are even worse) is not recommended as @SLaks mentions, but makes for some good regex learning 🙂 Also, regexes are not known for performance, especially a ginormous one.