we can use javascript code
function validateAlphaNumericCode(event) {
keyEntry = (event.which) ? event.which : event.keyCode
if (((keyEntry >= '65') && (keyEntry <= '90')) || ((keyEntry >= '97') && (keyEntry <= '122')) || (keyEntry == '37') || (keyEntry == '39') || (keyEntry == '46') || (keyEntry == '8') || (keyEntry == '9') || (keyEntry == '95') || ((keyEntry >= '48') && (keyEntry <= '57')))
return true;
else
return false;
}
validate this code with your textbox.