In Row databound event of gridview, you have to retrive all the values and assign it based on ddl selected value so that the state will remains same.
refer below links for rowdatabound event of gridview
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Explain concepts important to validation of email addresses with regular expressions. Applies to most languages/tools that uses regular expressions.
Consider email address RFCs:
Look at HTML5 form input element (with type=email) regex pattern: