Hi,

Taking your comments and improving the first patch a bit resulted in the attached one. It replaces the previous. Using string-prefix-p and string-suffix-p solves the out of index problem in the substrings.

BR
Gustav 

> -----Original Message-----
> From: Nicolas Goaziou [mailto:mail@nicolasgoaziou.fr]
> Sent: Wednesday, November 25, 2015 15:53
> To: Gustav Wikström <gustav@whil.se>
> Cc: emacs-orgmode@gnu.org; sgeorgii . <sgeorgii@gmail.com>
> Subject: Re: [O] [RFC] [PATCH] Changes to Tag groups - allow nesting and
> regexps
> 
> Gustav Wikström <gustav@whil.se> writes:
> 
> > Hmm, since the error was thrown when trying to look at indexes outside
> > of the string in (substring ... ), I don't see how isregexp can be
> > left as is. We have to make sure the substring-code is not evaluated
> > if the tag is empty. What am I missing?
> 
> Nothing, I was clear as mud.
> 
>   (cond
>    ((string= "" tag) (setq f1 '(not tags)))
>    ((and (equal "{" ...)
>          (equal "}"...))
>     ...)
>    (t ...))
> 
> Or even
> 
>  ((and (string-prefix-p "{" ...)
>        (string-suffix-p "}" ...)))
> 
> on master.
> 
> Regards,