isAlphabetical method
Returns a Validator that accepts a value that is comprised of only letters, or empty.
https://stackoverflow.com/questions/20690499/concrete-javascript-regular-expression-for-accented-characters-diacritics https://unicode-table.com/en/ accepts
null, "", " ", "foo", " foo", "foo ", "foo bar", " foo bar "
rejects
"0", "X Æ A-12",
Implementation
Validator isAlphabetical({
String message = "Validator.isAlphabetical",
}) {
var source = r"^([A-Za-zÀ-ÖØ-öø-ÿ]* *)*$";
return copy().matches(RegExp(source, unicode: true), message: message);
}