Regular expression for no more than two repeated letters/digits

Question

I have a requirement to handle a regular expression for no more than two of the same letters/digits in an XSL file.

• no space
• does not support special chars
• support (a-z,A-Z,0-9)
• require one of a-z
• require one of 0-9
• no more than 2 same letter/digits (i.e., BBB will fail, BB is accepted)

What I have so far

(?:[^a-zA-Z0-9]{1,2}) 

Answer 1

This regex will do it: ^(?!.*([A-Za-z0-9])\1{2})(?=.*[a-z])(?=.*\d)[A-Za-z0-9]+$

Here's the breakdown:

(?!.*([A-Za-z0-9])\1{2}) makes sure that none of the chars repeat more than twice in a row.

(?=.*[a-z]) requires at least one lowercase letter

(?=.*\d) requires at least one digit

[A-Za-z0-9]+ allows only letters and digits

EDIT : removed an extraneous .* from the negative lookahead

Answer 2

(Partial solution) For matching the same character repeated 3 or more times consecutively, try:

([a-zA-Z0-9])\1{2,} 

Sample matches (tested both here and here): AABBAA (no matches), AABBBAAA (matches BBB and AAA), ABABABABABABABA (no matches), ABCCCCCCCCCC (matches CCCCCCCCCC).