Validaciones con expresiones Regulares

A string with at least one character but not only white spaces

(.{1,})(\S+)(\s*)

Only the characters of the alphabet:

^[a-zA-Z]+

Everything instead of digits:

^[\D]+

Everything except the given string "mystring":

^((?!mystring).)*$

http://www.cis.upenn.edu/~matuszek/General/RegexTester/regex-tester.html