empty regular expressions don't work on the BSDs

[Andreas Vögele]

There are five tests in test-suite/tests/numbers.test that use an empty 
regular expression. Here's the first test:

   (pass-if-exception
    "Proper exception with 0 modulus"
    (cons 'numerical-overflow "") <-- empty RE
    (modulo-expt 17 23 0))

The problem is that on Mac OS X and the other BSDs regcomp() does not 
accept empty regular expressions. The BSD regex implementation is based 
on Henry Spencer's implementation. I don't know what POSIX says about 
empty REs but Henry Spencer's manual page says that "an empty string is 
not a legal RE" (see http://arglist.com/regex/regex3.html).

The tests in numbers.test succeed if the empty REs are replaced with 
".". Another option would be to handle empty REs in string-match.