Function: subregexp-context-p

Return non-nil if POS is in a normal subregexp context in REGEXP.
A subregexp context is one where a sub-regexp can appear.
A non-subregexp context is for example within brackets, or within a
repetition bounds operator `\{...\}', or right after a `\'.
If START is non-nil, it should be a position in REGEXP, smaller
than POS, and known to be in a subregexp context.