How to block symbol in username

Question

Please help guys. How to block symbol in username registeration only numbers and letters only. username min 3, max 8.. please give code work in mobilechat version php 5

Answer 1

Do you want to block to not write or to check username and give advise to the user when write symbols?

Answer 2

username registeration numbers and letters, min 3 max 8 length

Answer 3

yes but is not the same to check next to submit or in Real time dont let symbols

Answer 4

can you give code example?