conditional regex on multiline string in python

In that case, you can use this regex

board add 0/\d+ (?!aaa|bbb)

If the regex matches then the string is invalid.

Python Example

import re


strings = [
    """xxxx
xxxx
 board add 0/1 aaa
 board add 0/2 aaa
 board add 0/3 bbb
 board add 0/4 bbb
 board add 0/5 aaa
#""",
    """xxxx
xxxx
 board add 0/1 xxx
 board add 0/2 aaa
 board add 0/3 bbb
 board add 0/4 aaa
 board add 0/5 bbb
#""",
    """xxxx
xxxx
 board add 0/2 aaa
 board add 0/3 bbb
 board add 0/4 bbb
 board add 0/5 aaa
#"""
]

for string in strings:
    print(not bool(re.search(r"board add 0/\d+ (?!aaa|bbb)", string)))

Output

True
False
True

Explanation

re.search returns the matched chunk of the string by the given pattern. If any matching does not exist returns None. The solution is based on negating the valid strings. So if neither aaa nor bbb is followed after board add 0/1 then the string is invalid. The rest are passed as you described in your previous question. So, if the re.search returns any value but None, then the not bool(...) will convert the value to the expected result.

NOTE: I'm using not bool(...) as the string is valid if it does not contain the pattern.

We can just focus on board add 0/1 and ignoring other board add 0/x in this question. In fact, despite the negation, your current solution fits my need. I am just wondering why we need negation, and why my answer does not work.

The first (board add 0/1)? (?(1) (aaa|bbb)) I cannot understand what did you expect to match. The second regex is similar to my answer to your previous question. The third one is more close to the answer.

I changed the regex I was suggesting to your previous question.

^(?:(?!board add 0\/1).)*$|^.*?board add 0\/1 (?:aaa|bbb).*$

Now you can use re.match instead of re.search

...

for string in strings:
    print(bool(re.match(r"^(?:(?!board add 0\/1).)*$|^.*?board add 0\/1 (?:aaa|bbb).*$", string, re.S)))

Output

True
False
True

NOTE: There also used the re.S (singleline) flag.