Detecting a repeated sequence in a string

A challenge that came up during Advent of Code 2025 day 2, where one had to detect whether a string was made up of a repeated sequence: ABCABCABC.

I initially wrote the algorithm by hand, but wasn’t pleased with the performance nor by how elegant the solution was. After having solved it, I wanted to learn more and found this page listing a few option. This solution caught my attention:

bool(re.fullmatch(r"(.+)\1+", value))

I’m more used to using the re.match function, but in this case we need the re.fullmatch:

If the whole string matches the regular expression pattern, return a corresponding Match. Return None if the string does not match the pattern; note that this is different from a zero-length match.

I never really needed that, but it’s good to have found a use case for it. Hopefully, I will remember this trick next time I need it!