Suggest adding Spencer regex libraries

[gsstark]

Henry Spencer wrote multiple regex libraries which were historically important and are still relevant today: https://garyhouston.github.io/regex/

The one that survives in production code today are the Tcl library which is used in both Tcl and Postgres.

It would be very interesting to know how they fare as they have a number of different approaches from the other historical lineages of regex library, many of which have not been picked up by modern implementations.

The Postgres regex engine supports a lot of modern features like being able to turn on/off greediness per capture group and supporting unicode (well whatever the OS puts in wchar_t).

[mariomka]

It sounds good. If you want to contribute adding some libraries check the Contributing section in the readme.