Your code does not accomplish the required task. The goal is to return nil when there is no match, or the nth match if there is. >>> re.search('a(x)a', 'axa'

Your code does not accomplish the required task. The goal is to return nil when there is no match, or the nth match if there is. >>> re.search('a(x)a', 'axa').group(1) 'x' >>> re.search('a(x)a', 'aya').group(1) Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: 'NoneType' object has no attribute 'group' The idiomatic python for the job is: match = re.search("`([^']*)'", caller[0]) if match: match.group(1) else: None (or some nonsense involving try/except, which would be even worse).