Scanning parenthesis in string/comment

[Yuan Fu]

I’m writing an expand-region-like function, which expands region by logical 
entities, like word-at-point, list-at-point, etc. Say I have this string

"Filter out invalid regions in REGIONS regarding ORIG.
ORIG is the current position. Each region is (BEG . END).”

And point is at the opening parenthesis, how do I detect this balanced 
parenthesis pair and expand the region over it? syntax-pass and forward-list 
only works with lists outside of strings and comments, IIUC.

Yuan