String validateUsername(String value) {
 String pattern = r'^(([^<>()[]\.,;:s@"]+(.[^<>()[]\.,;:s@"]+)*)|(".+"))@(([[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}])|(([a-zA-Z-0-9]+.)+[a-zA-Z]{2,}))$';
RegExp regExp = new RegExp(pattern);
if (value.length == 0) {
  return "Username is Required";
} else if(!regExp.hasMatch(value)){
  return "Invalid username";
}else {
  return null;
}

TextField(
     inputFormatters: [
          BlacklistingTextInputFormatter(
            new RegExp("[^a-z^A-Z^0-9]+")) //Regex for accepting only alphanumeric characters
          ],
)

SELECT * FROM user WHERE user_name = "userName";