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To exclude a string of a particular size from a regular expression, you can use negative lookahead. For example, if you want to match all strings that are between 3 and 5 characters long but not the ones with the exact size of 4 characters, you can use the following regular expression:

^(?!.{4}$).{3,5}$

This negative lookahead expression (?!.{4}$) will exclude any string that has exactly 4 characters, and the rest of the regular expression will match any string between 3 and 5 characters long.