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.
Regional specific idioms do exist.
Here's a comment I left on an answer above:
成语 are not universal that is just not true & regional 成语 exist. There exist books like《鄂尔多斯方言成语词典》、《陕北方言成语3000条》、《忻州成语词典》, etc.
For instance you can check out this tieba post: 【来自《百家姓》的方言成语】 which mentions idioms like
周吴郑王 is also noted in《汉语大辞典》as: