in regex mode should catch most cases (replace with I).
Nothing's going to be perfect--like if there's markup dividing an actual word, for instance (not that I can think of any good reason why there should be).
But it should catch things like:
"T want to know."
or
There but for the grace of god go T.
But it will fail, if say, there's some dialog that attempts to simulate someone stuttering. Like:
"I want T-T-T-T-To know!"