(use-modules (ice-9 regex))
and see what happens.
>There seem to be a problem with some regex procedures not being
>"public enough":
>
>guile> match:substring
>#<procedure match:substring (match . args)>
>guile> (define-module (a))
>#<directory a 401de490>
>guile> match:substring
>ERROR: In expression match:substring:
>ERROR: Unbound variable: match:substring
>ABORT: (misc-error)