Validate a text box from database

hi all
 
I have a textbox that the user is suppose to enter a valid user name , the valid user names are stored in database. for example the valid user names might be  U100, U101, U102 ,U103,U104,U105,U106,U107. so if the user types anything other than that he should get a error message and I dont want to implement a drop down list for that , can anybody suggest me how to do this in asp or should i explore javascript for this.
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