Ahh that is an additional stipulation you did not initally make. Two of the options I gave meet this new criterion, also see the update I have added. However, IMO this is much like somebody coming here asking to implement some given functionality and saying "no modules". There is a very good reason this software(or modules) exist, and that is to meet a need and make the solution available for others, no reinventing the wheel.

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perl -pe "s/\b;([st])/'\1/mg"