Based on Quade's comment you can also use
\.\w{3-4}\W
where 3-4 means at least three but not more than four characters following a dot -
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actually, that will miss extensions at the very end of the line (that trailing \W means an actual non-word character) - instead, use
\.\w{3-4}\b
where \b just means a word boundary, which could be the end of the line
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