That is not | That's not | That isn't

all a correct.

All of the above are correct. English is flexible that way.

Thank you both for your quick responses!

You might want to learn more about when to use contractions. You can use contractions liberally in about 95% of documents. Many decades ago in formal use (and now in legal documents) frowned on the use of contractions.

Interesting, that's good to know. Thank you, Ragman.

Rag's observation, Greg, is highly pertinent to the esl who might seem confused by such issues