Highlight words of a given length

text = StringTake[ExampleData[{"Text", "AliceInWonderland"}], 400]

StringTake[ExampleData[{Text, AliceInWonderland}], 400]

Manipulate[TextCell[Row[List@@StringReplace[text, {RegularExpression["\\b\\w{" <> ToString[u] <> "}\\b"] :> Style["$0", 18, Bold]}]], "TR"], {u, 1, 10, 1}, SaveDefinitions -> True]

Manipulate[TextCell[Row[List @@ StringReplace[text, {RegularExpression[StringJoin["\\b\\w{", ToString[u], "}\\b"]] :> Style["$0", 18, Bold]}]], TR], {u, 1, 10, 1}, SaveDefinitions -> True]