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 s

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 https://github.com/k-takata/Onigmo/issues/87 I hope this helps you to understand the operator.