This is really a question about implementation rather than anything a user of GNU Emacs would be interested in.
Emacs regexps are represented as strings. There is a nice
rx DSL for constructing regexps out of s-expressions, but it produces a string not some other more fundamental data structure. I assume that the underlying C regexp engine uses something lower level than strings to represent a regexp matcher.
Does emacs parse / compile every regexp string every time a procedure is called, or is there some memoization going on there under the hood... e.g. an LRU of the regexp string to the underlying compiled data structure. Is it possible to customise any of the parameters for that cache as a user?