If you dig a bit, you can find WebUIValidation.js (mine is in /aspnet_client/system_web/1_0_3705_288 )<BR><BR>In this file, you can find the validation function below which will always return true (valid) if the input&#039;s value is only whitespace.<BR><BR>If you don&#039;t want to use paired required and regex validators, you can build your own regEx validator control from the custom validator or from the baseValidator<BR><BR>http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpgenref/html/cpconaspnetsyntaxforvalidationcontrols.asp<BR><BR> function RegularExpressionValidatorEvaluateIsValid(val) {<BR><BR> var value = ValidatorGetValue(val.controltovalidate);<BR> if (ValidatorTrim(value).length == 0)<BR> return true; <BR> var rx = new RegExp(val.validationexpression);<BR> var matches = rx.exec(value);<BR> return (matches != null && value == matches[0]);<BR>}