checkMask function
Implementation
void checkMask(String mask) {
if (_oneDashRegExp.hasMatch(mask)) {
throw ('A mask cannot contain more than one dash (-) symbols in a row');
}
if (!_startPlusRegExp.hasMatch(mask)) {
throw ('A mask must start with a + sign followed by a digit of a rounded brace');
}
if (!_maskContentsRegExp.hasMatch(mask)) {
throw ('A mask can only contain digits, a plus sign, spaces and dashes');
}
}