Tried solving today's #adventofcode using only regex, and it turns out that it's not easy to match a string containing any character consecutively repeated exactly 2 times (but no more).
For example, I want to match "abccd", but not "abcccd".
But if you want to make it generic, e.g. match any letter, you might try using backreferences:
This produces the error: "lookbehind assertion is not fixed length" since the regex engine cannot know how big the backref is.
@ihabunek this would have been my first thought
@hirojin I did think of something similar, but [\1] matches character with ASCII code 1 (base 80), and not the backref. E.g. [\80] would match 0.
As far as I can tell, it's not possible to use back-references in square brackets.
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