For example, if you do ListFiles[folder=~/Downloads] -> FilterString[field=filename, regex=\.pdf$], it will parse the regex, verify that it only uses features which are available in Spotlight's query syntax (roughly, a glob with slightly funny syntax), and rewrite that adjacent pair of operations into a single API call behind the scenes:
kMDItemFSName LIKE[c] "*.pdf"
I can report, having written parsers for a few common regex dialects, that there are a ton of obscure regex features, with different semantics everywhere. 100% conversion is almost never possible.A lot of the work in deciding if a translation is possible is in identifying if something is good enough, e.g., SQLite can't do case-insensitive search (in general), but if your regex happens to be /([0-9]+)/ then case-sensitive search will work just fine. Fortunately, for Strukt, if a conversion is impossible, I can just run the operations as written: it's much slower, but still correct.
I've thought about breaking this part of Strukt off into a tool just for regex editing, but that always seemed rather esoteric. Would there be any use or demand for that, do you think?
Some of the look arounds might be too powerful to do in some engines so it would not always work, but it would still be quite useful if it just handled the differences in escaping, specifying modes like case insensitivity or multiline matching, and referencing match groups in replacements.