'$' is undefined alert message is populated always when running the java script code

The following code always populates the "$ is undefined alert message in JavaScript". Please do help me out with this. Thanks in advance Code for Script tag:

function CheckString() {
     try{
     var s = "";
     s=$('#regExString').val();
     alert(s);
     var regExpression = /^[A-Z,a-z]\d[A-Z,a-z][\s{1}]?\d[A-Z,a-z]\d/;
     if (regExpression.test(s))
     alert("Valid postal code.");
     else
      alert("Invalid postal code.");
     } catch (e) {
     alert(e.message);
     }
     }

The html input text tag is passed as the text field and button has on click function CheckString which validates the regular expression