isAlphanumerical method
Returns a Validator that accepts a value that is comprised of only letters and numbers, or empty.
https://stackoverflow.com/questions/20690499/concrete-javascript-regular-expression-for-accented-characters-diacritics https://unicode-table.com/en/
Implementation
Validator isAlphanumerical({
String message = "Validator.isAlphanumerical",
}) {
var source = r"^([A-Za-zÀ-ÖØ-öø-ÿ0-9]* *)*$";
return copy().matches(RegExp(source), message: message);
}