Question

I am learning C# and
I have a small testing program where the console should receive a digit as input as and not an alphabetic character.

string inputString;

        string pattern = "[A-Za-z]*";
        Regex re = new Regex(pattern);

        inputString = Console.ReadLine();

        while(re.Match(inputString).Success)
        {
            Console.WriteLine("Please stick to numerals");
            inputString = Console.ReadLine();
        }
        Console.WriteLine(inputString);

Problem is the compiler doesn’t differentiate between an alphabetical character or a numeral.

Any suggestion perhaps
The code seems to be right.

Answer 1

The problem is that string pattern = "[A-Za-z]*"; will also match 0 characters because of the * quantifier.

If you only want to check if there is a letter in the string, just use

string pattern = "[A-Za-z]";

but of course this is only matching the ASCII letters. The better approach is to use Unicode properties

string pattern = @"\p{L}";

\p{L} will match any Unicode code point with the property “Letter”.

NOTE:

I hope you are aware that this is not checking for only digits, its checking if there is a letter in the input. This will of course accept characters that are not digits and not letters!

If you want to check for only digits you should go for @musefan’s answer or use regex this way

string inputString;

string pattern = @"^\p{Nd}+$";
Regex re = new Regex(pattern);

inputString = Console.ReadLine();

while (!re.Match(inputString).Success) {
    Console.WriteLine("Please stick to numerals");
    inputString = Console.ReadLine();
}
Console.WriteLine(inputString);

\p{Nd} or \p{Decimal_Digit_Number}: a digit zero through nine in any script except ideographic scripts.

See http://www.regular-expressions.info/unicode for more information about Unicode properties.

The next alternative is to check if there is “not a digit” in the input:

string pattern = @"\P{Nd}";
...
while (re.Match(inputString).Success) {

The you need to change only the pattern, \P{Nd} is the negation of \p{Nd} and will match if there is one non digit in the input.