k-takata/Onigmo

Implement Absent Operator

k-takata opened this issue · 5 comments

k-takata commented

Slide and paper by Akira Tanaka (@akr) (both Japanese):

TODO:

  • Syntax:
    Maybe the form of (?...) should be used. For example (?~...)?
  • Implementation:
    Is it easy?
k-takata commented

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.
Then absent operator can be a syntax sugar of (?:(?!expr)(?m:.))*?

k-takata commented

It seems that (?~expr) is different from (?:(?!expr).)*.
\A(?~abc)c should match abc (because (?~abc) matches ab), but \A(?:(?!abc).)*c doesn't match.

https://twitter.com/nalsh/status/809375869626150912

k-takata commented

topic/absent-operator branch was merged into the master branch.
Maybe more tests are needed, but closing for now.

k-takata commented

However @sinya8282 says that look-ahead assertion is a regular expression

This is discussed in a paper by Akimasa MORIHATA: