isHexString function
Is the string a hex string.
Implementation
bool isHexString(String value, {int length = 0}) {
if (!RegExp('^0x[0-9A-Fa-f]*\$').hasMatch(value)) {
return false;
}
if (length > 0 && value.length != 2 + 2 * length) {
return false;
}
return true;
}