L&S, http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0059%3Aentry%3Dex, wrote:ex or ē (ex always before vowels, and elsewh. more freq. than e; e. g. in Cic. Rep. e occurs 19 times, but ex 61 times, before consonants—but no rule can be given for the usage; cf., e. g., ex and e together: “qui ex corporum vinculis tamquam e carcere evolaverunt,” Cic. Rep. 6, 14. But certain expressions have almost constantly the same form, as ex parte, ex sententia, ex senatus consulto, ex lege, ex tempore, etc.; but e regione, e re nata, e vestigio, e medio, and e republica used adverbially; v. Neue, Formenl. 2, 756 sq.)...
Venabili wrote:Or can ex be used in all times besides the few listed in the L&S?
L&S,http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0059%3Aentry%3Dab, wrote:The second form of this preposition...was ab, which has become the principal form and the one most generally used through all periods—and indeed the only one used before all vowels and h; here and there also before some consonants, particularly l, n, r, and s; rarely before c, j, d, t; and almost never before the labials p, b, f, v, or before m, such examples as ab Massiliensibus, Caes. B. C. 1, 35, being of the most rare occurrence.