Re: efficient regular expression that never matches

[cinsk <cinsky@gmail.com>]

On Saturday, November 21, 2015 at 12:59:51 AM UTC+9, John Wiegley wrote:
> >>>>> Stefan Monnier <monnier at iro.umontreal.ca> writes:
> 
> >> I'm thinking of "$a", but it should scan all the string until end of the
> >> string, so it may not be efficient.
> 
> > IIRC I used something like "\\`\\'a".
> 
> Yes, also because "$" only has special meaning as the last character of the
> regexp, whereas \\' should always mean "end of <context>".
> 
> John

Yes, I agree with that,

However, "$a" means that the regular expression engine may read all strings until the end of the line.  So, if I can do something like "^<SOME CHARACTER OR CONDITION IMPOSSIBLE>", then the engine should detect before reading all characters of the line, I presume.  Perhaps "^\uFFFFF" can be a solution?  Since \uFFFF is noncharacter of the unicode (\uFFFF can be use for the internal purpose but generally not used.)