String email="demoemail@gmail.com";
var nameuser = email.split("@");
var emailcaracter=email.replaceRange(2,nameuser[0].length,"*" * (nameuser[0].length-2));
print(emailcaracter);

结果：

de*******@gmail.com

您可以使用此方法来实现您确切想要的结果：

String unconsore(String string, {int consoreLevel = 2}) {
  final parts = string.split("@");

  if (parts.length != 2) {
    // Handle the case where there is no "@" separator in the string
    return string;
  }

  final stringBeforeA = parts[0];
  final stringAfterA = parts[1];

  final unconsoredBeforeA = stringBeforeA.replaceRange(
      consoreLevel, stringBeforeA.length - consoreLevel, "*" * (stringBeforeA.length - consoreLevel * 2));

  return "$unconsoredBeforeA@$stringAfterA";
}

// example
print(unconsore("namesurname@mail.com")); // na*******me@mail.com