are there region specific idioms? I am trying to learn more about idioms. I was told this might be a good place to ask this question, so please delete this if this isn't good for this exchange. Are there widely used idioms specific to Taiwan or Hong Kong but not used on the mainland or overseas? Thank you in advance.
中國成语 (Chinese idioms) are universal because they are all literary phrases. All Chinese no matter what dialect they are speaking  write in Standard Written Chinese (SWC)

On the other hand, common expressions or folk sayings are very regional because they are mostly colloquial phrases. 

Some popular expression in one dialect would be picked up by near by regions. For example, many Hong Kong Cantonese created expressions are understood and used by many mainland Chinese in the nearby regions. 

See 成语 versus 俗语 versus 谚语, what is the difference?