Validate only if data filled

Validate only if data filled

Well, I need to implement this criteria in one of the form field in my
application, the rules goes like:-
input_field,Criteria=>It needs to be validated only if the "input_field"
has values in it, if its empty no need to validate, well the thing is I
have fixed up the "second-validation" method for that field, i.e to check
digits and symbols and bla bla bla.....
But the problem is I need to apply the second validation only if the field
is filled. I couldn't come up with any solution, since being new to
javascript and all.
And I am using JQuery validator, for the form.