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Akira's paper says that look-ahead assertion doesn't have theoretical backgrounds. However @sinya8282 says that look-ahead assertion is a regular expression (in context of formal language theory), and it can be proved by using boolean automaton.
It seems that
(?~subexp) matches any string which doesn't have subexp as its suffix. This is based on Tanaka Akira's paper and slide: * https://staff.aist.go.jp/tanaka-akira/pub/prosym49-akr-paper.pdf * https://staff.aist.go.jp/tanaka-akira/pub/prosym49-akr-presen.pdf
This is discussed in a paper by Akimasa MORIHATA:
* Support absent operator k-takata/Onigmo#82 * https://github.com/k-takata/Onigmo/blob/Onigmo-6.1.1/HISTORY git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57603 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
Merge Onigmo 6.1.1 * Support absent operator k-takata/Onigmo#82 * https://github.com/k-takata/Onigmo/blob/Onigmo-6.1.1/HISTORY git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@57957 b2dd03c8-39d4-4d8f-98ff-823fe69b080e