把东西放在第n个数字之后，而不是第n个字符？

Question

I'm working on an autocomplete for SSN-like numbers in PHP. So if the user searches for '123', it should find the number 444123555. I want to bold the results, thus, 444<b>123</b>555. I then, however, want to format it as an SSN - thus creating 444-<b>12-3</b>555.

Is there some way to say 'put the dash after the nth digit'? Because I don't want the nth character, just the nth digit - if I could say 'put a dash after the third digit and the fifth digit, ignoring non-numeric characters like <, b, and >' that would be awesome. Is this doable in a regex?

Or is there a different method that's escaping me here?

Answer 1

Just iterate over the string and check that each character is a digit and count the digits as you go.

That will be so much faster than regex, even if regex were a feasible solution here (which I am not convinced it is).