In the small language you can test library availability with:
(define-library (foo bar)
...
(cond-expand
((library (scheme regexp))
...)
(else
...)))
but you might want to be sure that the regexp-unicode feature was available, like this:
(define-library (foo bar)
...
(cond-expand
((library-provides (scheme regexp) regexp-unicode)
...)
(else
...)))