isCharDigitOrDecimalPoint function
Returns true if char represents a digit or a decimal point,
false otherwise.
This function checks whether the input string is not empty and whether its first character is a digit (i.e., has a Unicode code point between the code points of '0' and '9') or a decimal point ('.').
If the input string satisfies either of these conditions, the function
returns true. Otherwise, it returns false.
Implementation
bool isCharDigitOrDecimalPoint(String char) {
if (char.isEmpty) return false;
final isDecimalPoint = char == '.';
return isCharDigit(char) || isDecimalPoint;
}