Simple Reg-Exp...

Two simple regexp.......
for match string pippo.pluto
^.*(?:pippo.pluto).*$

or for not match string pippo.pluto
^((?!pippo.pluto).)*$

Useful links....
to test.....
http://www.regular-expressions.info/javascriptexample.html
to search ready-to-use reg expressions....
http://regexlib.com