Although general PCRE syntax has clearly won, that’s distinct from the infinite-memory-and-time matter. Engines that don’t use backtracking and can search in linear time (O(regex-size × search-text-size)) have been making a serious comeback in recent years, at only the cost of not supporting lookaround assertions or backreferences. For example, the re2 library, or Rust’s regex crate, both of which are very popular.
Alas that approach only works well for your basic regular expressions that support Kleene-Star, concatenation and union.
Regular languages are also closed under intersection and taking prefix etc (see https://en.wikipedia.org/wiki/Regular_language#Closure_prope...), and if you want to expose those operators in your regular expressions, Russ Cox's technique doesn't really work.
I suspect it should be possible to transform two intersected (K*,C,U) expressions into a new (K*,C,U) expression by walking over the two, dropping incompatible terms, reducing sets, and redistributing and expanding sub-expressions to combine the two, though I do not have time to take a serious look at this at the moment.
I also suspect that this might result in a combinatorial explosion of fused terms, but that's irrelevant as we're only worried about whether intersection can be implemented in Russ Cox's method, not whether a regular expression might require several earths or so worth of RAM to hold :)
Nothing in the wiki article mentions taking a prefix, but that just sounds like `(prefix|)`?
You can still explicitly build the finite automaton for your closure-extended regular language. That one might be big, but matching is still linear in the length of the text you are searching through.
> Nothing in the wiki article mentions taking a prefix, but that just sounds like `(prefix|)`?
They implicitly mention prefixes, when they talk about 'the trio operations: string homomorphism, inverse string homomorphism, and intersection with regular languages.' If I remember right, the prefix operation is:
Take a language L, restrict it to the strings that have the right prefix: `L' := L & (prefix.*)` then chop off that prefix from every member of L'.
