how to match a group earlier in the regex?

[Admiralfeb]

Hi, I'm attempting complete one of the Advent of code puzzles back from 2015 and it's asking to find a pair of characters that repeats at least twice without overlapping in the input.

I'm attempting to use regex for this and the string r"\b(\w{2}).*?\1\b" seems to be the right path, but the regex fails saying that backreferences are not supported.

Am I missing something on how to handle making sure that the capture group is listed twice in the string?

[BurntSushi]

The very first paragraph of the crate docs has your answer, emphasis mine:

This crate provides routines for searching strings for matches of a regular expression (aka “regex”). The regex syntax supported by this crate is similar to other regex engines, but it lacks several features that are not known how to implement efficiently. This includes, but is not limited to, look-around and backreferences. In exchange, all regex searches in this crate have worst case O(m * n) time complexity, where m is proportional to the size of the regex and n is proportional to the size of the string being searched.

You'll either need to use a different regex engine, not use regexes, or match more than you need and filter out matches after the fact based on checking group equality.