C# Regex Validation Rule using Regex.Match()

Question

I've written a Regular expression which should validate a string using the following rules:

1. The first four characters must be alphanumeric.
2. The alpha characters are followed by 6 or 7 numeric values for a total length of 10 or 11.

So the string should look like this if its valid:

CCCCNNNNNN or CCCCNNNNNNN

C being any character and N being a number.

My expression is written: @"^[0-9A-Za-z]{3}[0-9A-Za-z-]\d{0,21}$";

My regex match code looks like this:

var cc1 = "FOOBAR"; // should fail.
var cc2 = "AAAA1111111111"; // should succeed

var regex = @"^[0-9A-Za-z]{3}[0-9A-Za-z-]\d{0,21}$";

Match match = Regex.Match( cc1, regex, RegexOptions.IgnoreCase );

if ( cc1 != string.Empty && match.Success )
{
     //"The Number must start with 4 letters and contain no numbers.",
     Error = SeverityType.Error
}

I'm hoping someone can take a look at my expression and offer some feedback on improvements to produce a valid match.

Also, am I use .Match() correctly? If Match.Success is true, then does that mean that the string is valid?

Answer 1

The regex for 4 alphanumeric characters follows by 6 to 7 decimal digits is:

var regex = @"^\w{4}\d{6,7}$";

See: Regular Expression Language - Quick Reference

---

The Regex.Match Method returns a Match object. The Success Property indicates whether the match is successful or not.

var match = Regex.Match(input, regex, RegexOptions.IgnoreCase);

if (!match.Success)
{
    // does not match
}

Answer 2

The following code demonstrates the regex usage:

        var cc1 = "FOOBAR"; // should fail.
        var cc2 = "AAAA1111111"; // should succeed
        var r = new Regex(@"^[0-9a-zA-Z]{4}\d{6,7}$");
        if (!r.IsMatch(cc2))
        {
            Console.WriteLine("cc2 doesn't match");
        }
        if (!r.IsMatch(cc1))
        {
            Console.WriteLine("cc1 doesn't match");
        }

The output will be cc1 doesn't match.

Answer 3

The following code is using a regular expression and checks 4 different patterns to test it, see output below:

using System;
using System.Text.RegularExpressions;   
public class Program
{
    public static void Main()
    {
        var p1 = "aaaa999999"; 
        CheckMatch(p1);
        p1 = "aaaa9999999";
        CheckMatch(p1);
        p1 = "aaaa99999999";
        CheckMatch(p1);
        p1 = "aaa999999";
        CheckMatch(p1);
    }

    public static void CheckMatch(string p1)
    {
        var reg = new Regex(@"^\w{4}\d{6,7}$");
        if (!reg.IsMatch(p1))
        {
            Console.WriteLine($"{p1} doesn't match");
        }
        else
        {
            Console.WriteLine($"{p1} matches");
        }
    }
}

Output:

aaaa999999 matches
aaaa9999999 matches
aaaa99999999 doesn't match
aaa999999 doesn't match

Try it as DotNetFiddle