Match Substring After Replacement LeetCode solution in Python

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from collections import defaultdict
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import re
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class Solution:
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    def matchReplacement(self, s: str, sub: str, mappings: List[List[str]]) -> bool:
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        reachable = defaultdict(set)
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        for a, b in mappings:
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            reachable[a].add(b)
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        for c in sub:
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            reachable[c].add(c)
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        regex = ""
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        for c in sub:
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            if len(reachable[c]) > 1:
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                regex += "("
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                regex += "|".join(reachable[c])
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                regex += ")"
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            else:
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                regex += c
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        return re.compile(regex).search(s)