It is possible to index content and then retrieve it with (simple) regular expressions, see (https://swtch.com/~rsc/regexp/regexp4.html https://swtch.com/~rsc/r

It is possible to index content and then retrieve it with (simple) regular expressions, see (https://swtch.com/~rsc/regexp/regexp4.html https://swtch.com/~rsc/regexp/regexp4.html). The trick is that each regular expressions can be seen as a tree where leaves are n-grams to be searched for, and nodes are AND and OR logical operations of n-grams and intermediate nodes. So, a regexp like /banana/ can be seen as a search for the trigrams "ban" and "ana" and "nan" and "ana", all in that order. A regexp like /ca[tp]/ can be seen as a search for "cat" or "cap". Once you have simple n-grams like that it's trivial to index content on the way in (note that what you index doesn't actually need to interpret content, all you need is to split every n characters) and use that index to restrict potential matches.