Validate user input using C# RegularExpressions

Question

How to Validate numerical input, max length limited 3 and should not start with zero characters (010, 001 should invalidate). I used C# regex.IsMatch() with following regex ([1-9]|[1-9][0-9]|[1-9][0-9][0-9])* . But it validating inputs start with zeros. How to resolve this..?

Answer 1

You can omit the alternations by repeating a character class 0 - 2 times and you should use anchors to assert the start ^ and the end $ of the string.

^[1-9][0-9]{0,2}$

• ^ Start of string
• [1-9] Match a digit 1-9
• [0-9]{0,2} Match 0, 1 or 2 times a digit 0-9
• $ Assert end of the string

Usage:

bool isMatch = Regex.IsMatch("100", @"^[1-9][0-9]{0,2}$");

Regex demo

Answer 2

Regex are great sure. Note that you could achieve this easily without any regex:

static bool IsValid(string input)
{
    return !string.IsNullOrEmpty(input)
        && input.Length < 3 && !input.StartsWith("0") && input.All(char.IsDigit); 
}

Try it online

Answer 3

I'd be more specific with this one:

^((?!(0))[0-9]{0,3})$ 

Explanations:

• 1st one is not 0 & i want maximum 3 digits between 0 and 9.

Answer 4

[1-9][0-9]{2}$

This would work as per your requirement. testes on regex buddy with following test cases

1. 001 Fail
2. 1000000 Fail
3. 900 Pass
4. 010 fail

Answer 5

您可以使用^[1-9][0-9]{0,2}$ ，不允许以零开头