≪xiehouyu≫, witty folk simile (popular witty or humorous simile consisting of two parts, of which the second is the "punch line" and is often left unsaid because the listener is assumed to be familiar with it)
We have this in English as well:
•Make like a banana—and split
•Make like a tree—and leave
(Although I'm not sure what we call these in English)
A Chinese-English Dictionary
a two-part allegorical saying, of which the first part, always stated, is descriptive, while the second part, sometimes unstated, carries the message, e.g. 泥菩萨过河-自身难保 like a clay idol fording a river-hardly able to save oneself (let alone anyone else)
The Chinese Wikipedia entry for xiehouyu has a 粤语常见歇后语 and 客家话歇后语 sections - so while some xiehouyu certainly are regional - and every topolect has them - the ones in dictionaries or that are "well known" will be understandable everywhere.