Question

I’ve been struggling to figure out how to best do this regular expression.

Here are my requirements:

• Up to 8 characters
• Can only be alphanumeric
• Can only contain up to three alpha characters [a-z] (zero alpha characters are valid to)

Any ideas would be appreciated.

This is what I’ve got so far, but it only looks for contiguous letter characters:

^(\d|([A-Za-z])(?!([A-Za-z]{3,}))){0,8}$

Answer 1

I’d write it like this:

^(?=[a-z0-9]{0,8}$)(?:\d*[a-z]){0,3}\d*$

It has two parts:

• (?=[a-z0-9]{0,8}$)
  • Looksahead and matches up to 8 alphanumeric to the end of the string
• (?:\d*[a-z]){0,3}\d*$
  • Essentially allowing injection of up to 3 [a-z] among \d*

---

Rubular

On rubular.com

12345678    // matches
123456789
@(#*@$
12345       // matches
abc12345
abcd1234
12a34b5c    // matches
12ab34cd
123a456     // matches

---

Alternatives

I do think regex is the best solution for this, but since the string is short, it would be a lot more readable to do this in two steps as follows:

• It must match [a-z0-9]{0,8}
• Then, delete all \d
  • The length must now be <= 3