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Diffstat (limited to 'FAQ.md')
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@@ -157,13 +157,37 @@ tool. With that said, How do I use lookaround and/or backreferences? </h3> -This isn't currently possible. ripgrep uses finite automata to implement -regular expression search, and in turn, guarantees linear time searching on all -inputs. It is difficult to efficiently support lookaround and backreferences in -finite automata engines, so ripgrep does not provide these features. +ripgrep's default regex engine does not support lookaround or backreferences. +This is primarily because the default regex engine is implemented using finite +state machines in order to guarantee a linear worst case time complexity on all +inputs. Backreferences are not possible to implement in this paradigm, and +lookaround appears difficult to do efficiently. -If a production quality regular expression engine with these features is ever -written in Rust, then it is possible ripgrep will provide it as an opt-in +However, ripgrep optionally supports using PCRE2 as the regex engine instead of +the default one based on finite state machines. You can enable PCRE2 with the +`-P/--pcre2` flag. For example, in the root of the ripgrep repo, you can easily +find all palindromes: + +``` +$ rg -P '(\w{10})\1' +tests/misc.rs +483: cmd.arg("--max-filesize").arg("44444444444444444444"); +globset/src/glob.rs +1206: matches!(match7, "a*a*a*a*a*a*a*a*a", "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"); +``` + +If your version of ripgrep doesn't support PCRE2, then you'll get an error +message when you try to use the `-P/--pcre2` flag: + +``` +$ rg -P '(\w{10})\1' +PCRE2 is not available in this build of ripgrep +``` + +Most of the releases distributed by the ripgrep project here on GitHub will +come bundled with PCRE2 enabled. If you installed ripgrep through a different +means (like your system's package manager), then please reach out to the +maintainer of that package to see whether it's possible to enable the PCRE2 feature. |