alphabets == alpha_simple_capital??
you mean without numbers, slashes, commas ?
if its that you need to make a JS script that runs at the text change so that if the keyCode is not among the 'alphabet' values then its not added to the textbox
or simply a php script which detect it as an error and show the error message
php has isNumeric() and isNaN() functions you could use to check if its a number. Then create an array with all special characters and loop through it to see if the content contains them or use regex to perform a search for numbers and special characters. Its best if you did this on the client end though since it wont burn server resources doing it.