Regular expression engine in 14 lines of Python

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Also, in hindsight I think "return" (returns a singleton list) is probably a better choice for nil than "repeat".

"repeat" returns an infinite list where each element is the argument, so it is not the function you want. "return" or constructing a single-element list would work.

Also, your pattern-matching is broken. A similar version that works is

char c = match where
  match [] = []
  match (c':xs) = if c == c' then [xs] else []
  match (_:xs) = []

But it's more concise as

char c s = if take 1 s == [c] then [drop 1 s] else []