> By definition submatches 1 and 2 exist when RES is true.
> Thus, I’d remove ‘safe-match:substring->number’ and do:

>   (match (string-match "^([0-9]+)\\.\\.([0-9]+)$" str)
>     (#f #f)
>     (matches
>      (let ((start (number->string (match:substring matches 1)))
>            (end   (number->string (match:substring matches 2))))
>        ...)))

Done.

> Probably this can reduce to a big ‘cond’, which would be even more
> readable:

>   (cond ((maybe-integer)
>          =>
>          list)
>         ((string-match "^([0-9]+)\\.\\.([0-9]+)$" str)
>          =>
>          (lambda (match)
>            ...))
>         ...)

Are you sure?  I haven’t found a way to make ‘cond’ as readable as ‘or’.

I’m attaching a sketchy version.  If you don’t see any problems, I’ll
try to integrate this code into ‘package.scm’.

(Something is wrong with the store on my machine, so I can’t properly
test the filtering part.  But I’ll do it as soon as possible.)