Minimum eight characters, at least one letter and one number:
^(?=.*[A-Za-z])(?=.*\d)[A-Za-z\d]{8,}$
Minimum eight characters, at least one letter, one number and one special character:
^(?=.*[A-Za-z])(?=.*\d)(?=.*[$@$!%*#?&])[A-Za-z\d$@$!%*#?&]{8,}$