> Le 19 nov. 2015 à 20:17, Nozomu Katō <noz...@akenotsuki.com> a écrit : > > On Thu, 19 Nov 2015 09:04:41 +0100, Yang Guo wrote: >> This implementation supports variable length lookbehind similar to .NET's >> semantics. It does so by emitting code to read backwards inside the >> lookbehind. The size of the change without platform ports and tests is >> about 600 lines. > > When regular expressions inside a look-behind are evaluated backwards, > the automaton will not have a chance to know what has been captured in > $1 in /(?<=(.)$1)/, since the automaton reaches $1 prior to (.). > > This can be fixed by defining that a backreference referring to a > capture inside the same look-behind has to be put leftwards contrary to > the usual rule, but saying so would impose reading regular expressions > backwards or prefetching forward expressions inside a look-behind upon > the parser, to know if the corresponding capturing parentheses exist. > > I am not sure if it is easy or there is a simpler solution, anyhow, if > ECMAScript supports .NET-like look-behinds this matter would need to be > fixed. > > Nozomu
There is always the coward solution: forbid backreferences inside lookbehinds. If we are going to ask programmers to write /(?<\1(.))/, it may be a good idea to prevent footguns like /(?<(.)\1)/ by throwing a syntax error (for it can be statically determined that it will not work as intended). —Claude _______________________________________________ es-discuss mailing list es-discuss@mozilla.org https://mail.mozilla.org/listinfo/es-discuss
