Re: [mind game] Seeking for simple task impossible via regexps

[Coda Highland <chighland@...>]

On Fri, Feb 24, 2017 at 1:29 AM, Martin <eden_martin_fuhrspam@gmx.de> wrote:
> Hello all!
>
> I'm seeking for simply formulated and easy-coding string matching
> task which is impossible to solve via currently existing regular
> expressions handlers.
>
> (Indeed I'm seeking for a complete definition of task classes that
> regexps can't handle in principle.)
>
> For example:
>
>   Check that given string in lowercase contains no more than one entry
>   of alphabet character.
>
>   (so all permutations of subset of alphabet [a-z] should be matched)

Not hard at all. If you support backreferences in pattern strings
(common feature) then it's just /([a-z]).*\1/. If you don't, then it's
more verbose, but it's just /(a.*a|b.*b|c.*c| --(snip)-- |y.*y|z.*z)/.

> Another example:
>
>   Check that given string in lowercase contains all symbols of
>   predefined alphabet. Ignore spaces.
>
>   (so "the quick brown fox jumps over the lazy dog" should match)

Also quite tractable, using lookahead: /(?=.*a)(?=.*b) --(snip)-- (?=.*z)/.

> My bet is that no existing regexp facilities may handle this
> tasks. But maybe some of them can, so asking.
>
> (I assume that calling user-supplied function from regexp handler
> is not allowed. Or else this is trivially solved by Lua's gsub().)
>
> -- Martin

/s/ Adam