Form Validation
Using Regular Expressions

function ValidateInput(form) {
     var LB = "\n";
     var msg = msg1 = msg2 = "";
     var msghdr1 = "The following fields are missing:" + LB + LB;
     var msghdr2 = "The following fields are invalid:" + LB + LB;
     var msgftr = LB + "Please complete and try again.";

/*
     var valid1stName = /^[a-z]+$/;

     if (!form.First.value) msg1 += "First Name" + LB;
     else if (!valid1stName.test(form.First.value.toLowerCase())) msg2 += "First Name" + LB;
*/

    var valid1stName = /^[a-z]+$/i;

     if (!form.First.value) msg1 += "First Name" + LB;
     else if (!valid1stName.test(form.First.value)) msg2 += "First Name" + LB;

     // Hays-Jones, O\'Brien, St\.\sJohn, 
     var validLastName = /^[-a-z\s\'\.]+$/i;

     if (!form.Last.value) msg1 += "Last Name" + LB;
     else if (!validLastName.test(form.Last.value)) msg2 += "Last Name" + LB;

     if (!form.Address.value) msg1 += "Address" + LB;

     // San\sJose, St\.\sLouis
     var validCity = /^[-a-z\s\.]+$/i;

     if (!form.City.value) msg1 += "City" + LB;
     else if (!validCity.test(form.City.value)) msg2 += "City Name" + LB;

     if (!form.State.selectedIndex) msg1 += "State" + LB;

     var zip = /^\d{5}(-\d{4})?$/

     if (!form.Zip.value) msg1 += "Zip Code" + LB;
     else if (!zip.test(form.Zip.value)) msg2 += "Zip Code" + LB;

     var age = /^\d{1,2}$/

     if (!form.Age.value) msg1 += "Age" + LB;
     else {
          if (isNaN(form.Age.value)) msg2 += "Age has to be a number" + LB;
          else if (!age.test(form.Age.value)) msg2 += " Age should be between 1 and 100" + LB;
     }

     if (!form.Income.selectedIndex) msg1 += "Income" + LB;

     if (!form.Sex[0].checked && !form.Sex[1].checked) msg1 += "Sex" + LB;

     var validEmail = /^[a-z][-a-z0-9\._]+@([-a-z0-9\._]+\.)+[a-z]{2,3}$/i;                                

     if (!form.Email.value) msg1 += "Email Address" + LB;
     else if (!validEmail.test(form.Email.value)) msg2 += "Email Address" + LB;

     if (msg1.length > 0 || msg2.length > 0) {
          if (msg1.length > 0) msg += msghdr1 + msg1;
          if (msg2.length > 0 && msg.length > 0) msg += LB + msghdr2 + msg2;
          else if (msg2.length > 0) msg += msghdr2 + msg2;

          alert(msg + msgftr);
          return false;
     }
     else return true;
}

</SCRIPT>