Onigmo

>Ruby even supports Perl regular expressions No, Ruby Regexp is based on the https://github.com/k-takata/Onigmo library. There are plenty of differences compared to Perl, for example `^` and `$` anchors always match start/end of lines without needing a flag, subexpression syntax uses `\g` instead of `(?N)` and so on.

You're probably thinking of PCRE, although Ruby actually uses Onigmo: https://github.com/k-takata/Onigmo

Ruby used Oniguruma (and now Onigmo) for RegEx https://github.com/k-takata/Onigmo

Hi, I'm the author of Onigmo regex library. I'm not sure that negative look-ahead/behind are formally regular expressions. (There is a (Japanese) paper which says that they are formally regular expressions, but I don't understand it.) However, the absent operator is formally regular expression. I translated the main points of Tanaka Akira's paper very roughly. https://github.com/k-takata/Onigmo/issues/87 I hope this helps you to understand the operator.

I'm still only a little way into analyzing it, but here's what I wrote for my own summary so far: Along with the usual tweaks, Onigmo has been updated (and now supports a new operator), OpenSSL (2.0.2 to 2.0.3), RubyGems (2.6.8 to 2.6.10), a race between read and close has been fixed, fixing including multiple proc blocks as literal hash values, and more. I'm particularly interested to work out what the new 'absent operator' in regexes is for and am studying it now: https://github.com/k-takata/Onigmo/issues/82