Question

i am searching for a regex for a string containing ‘a’ and ‘b’ that has the following two attributes:
1: The String has an Even number of characters
2: The String may not contain ‘aa’

Answer 1

It can be easily done with Perl-compatible regular expressions:
^(ab|bb|(ba(?!a)))*$

Basically it says that string must consist of ab, bb, ba substrings mixed in any order, BUT ba cannot follow another a character.

The string will have even length because all these subexpressions have even length. aa can’t appear in a string, because the only way for it to appear is in substring baab, but the regex specifically restricts ba to be followed by a.