Textbox with the single byte and Multibyte

Hi..

I have one textbox in Aspx page, in that the max length of textbox is 40, and and it should allow 40 char of english char and it should allow 20 char of type multibyte char like japanese or other than english.

so i need to validate the textbox text whether it is single char or multichar, if it is multichar and if the length is greater than 20 i need to display alert message.

How to carryout this??
any body know how to check the textbox text like is it single or multi byte char????