Scan values in a field and if they are all letters then alert. The second block of code does the same but for numbers

//alert on finding all letters
var noalpha = /^[a-zA-Z]*$/;
if (noalpha.test(document.formname.fieldname.value)) {
alert("Please enter at least one number in the \"username\" field.");
return false;
}

//alert on finding all numbers
var nonums = /^[0-9]*$/;
if (nonums.test(document.formname.fieldname.value)) {
alert("Please enter at least one letter in the \"username\" field.");
return false;
}