hello.
thougj i haven't tried it, i assume that you can add a handler to the onblur event. this should be called before the page is subtmitted to the server (i'm assuming that you're setting the autpostback of the textbox controlo to true and that itt's placed inside a updatepanel).
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Regards,
Luis Abreu
email: labreu_at_gmail.com
EN blog:http://msmvps.com/blogs/luisabreu