How to check if a username has 3 digits

Question

I want the user to only be able to proceed if their username's last 3 characters are digits. How would you code that? Any help would be appreciated!

My current code but it doesn't work:

static bool checkUsername(string user)
        {
            //bool containsAtLeastThreeDigits = user.Any(char.IsDigit);
            var counter = user.Count(x => Char.IsDigit(x));
            bool valid = false;
            int count = 0;

            if (counter == 3)
            {
                count++;
            }
            else
            {
                Console.WriteLine("Password must have 3 digits");
            }

            if (count == 1)
            {
                valid = true;
            }
            return valid;
        }

Please post any attempt you have made. Here's a few hints to get you going. You want to get the last 3 characters of the `string` username (https://stackoverflow.com/questions/6413572/how-do-i-get-the-last-four-characters-from-a-string-in-c/6413598) then check to see if these characters are a number (https://learn.microsoft.com/en-us/dotnet/api/system.int32.tryparse?view=netcore-3.1), you will also need to check to make sure that the username is at least 3 characters in length. — Ryan Wilson, Sep 30 '20 at 20:36
First I'd check to make sure the username is greater than or equal to 3 characters, else it's not going to pass the test, then if greater than or equal to 3 characters in length, get the last 3 characters of the username using `.SubString()`, then use `Int32.TryParse` to check if the last 3 characters make a number. I linked to articles covering `SubString()` and `TryParse()` above. If you still need help, let me know. — Ryan Wilson, Sep 30 '20 at 20:43
Your question states that you want the last 3 characters to be integers. But from your code sample, it looks like you were trying to just make sure that *any* three were integers. — Casey Crookston, Sep 30 '20 at 20:52

score 1 · Answer 1 · answered Sep 30 '20 at 20:47

1

You can simply check if the Length is greater than 2 characters, and then check that the substring consisting of the last 3 characters are all digits:

public static bool Last3AreDigits(string input)
{
    return input?.Length > 2 && input.Substring(input.Length - 3).All(char.IsDigit);
}

answered Sep 30 '20 at 20:47

Rufus L

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There's no real reason to allocate a substring when you can just check the last 3 digits using C# indexing. – MikeJ Sep 30 '20 at 21:11
@MikeJ Sure, that would also work. Same with `TakeLast(3)`. – Rufus L Sep 30 '20 at 21:16

John H · Answer 2 · 2020-09-30T21:14:48.207

Edit: I've just seen Casey Crookston's comment about the ambiguity of your question. So, if you do want to check to see if the input has at least 3 digits in any position, you can use the following:

using System.Linq;

static bool checkUsername(string user)
{
    return user?.Count(char.IsDigit) >= 3;
}

Original answer (checking the last 3 characters are digits)

You could do this in a couple of ways that are simpler than what you're currently doing.

using System.Linq;

static bool checkUsername(string user)
{
    return user?.Length >= 3 && user.TakeLast(3).All(char.IsDigit);
}

or

using System.Text.RegularExpressions;

static bool checkUsername(string user)
{
    return user != null && Regex.Match(user, @"\d{3}$").Success;
}

\d denotes a digit, the {3} specifies 3 occurrences of the preceding expression (i.e. 3 digits), and $ means match at the end of the string.

MikeJ · Answer 3 · 2020-09-30T21:11:47.903

Easiest option is to verify the length greater than some min and then use the new C# range indexing. I'm sure there's probably other validation you would want to do. But just the requirements you stated would look like this:

    static readonly (string,bool)[] UsernamesTestData = new (string,bool)[]
        {
            ("good123",true),
            ("bad12",false),
            ("bad1",false),
            ("no",false),
            (" ",false),
            ("",false),
            (null,false),
        };

    static void Main(string[] _)
    {
        foreach (var (username, expected) in UsernamesTestData)
        {
            bool isValid = IsValidUsername(username);

            Console.WriteLine($"{username}: expected: {expected} tested as: {isValid}");
        }
    }

    static bool IsValidUsername(string name)
    {
        if (string.IsNullOrWhiteSpace(name) || name.Length < 4)
            return false;

        return char.IsDigit(name[^1]) && char.IsDigit(name[^2]) && char.IsDigit(name[^3]);
    }

Gave you a plus one for your good catch on my answer and your working solution. — Ryan Wilson, Sep 30 '20 at 21:28
Hey Mike, I think you wanted `name.Length < 3)` not `name.Length < 4)` — Ryan Wilson, Oct 01 '20 at 12:23
@RyanWilson no, the 4 was intentional. He wanted it to end in 3 digits which I took to mean there needed to be one other additional character. "a123". — MikeJ, Oct 14 '20 at 12:59

Ryan Wilson · Accepted Answer · 2020-09-30T21:37:15.803

0

Since everyone else is using Linq and/or Regex, here's a solution using SubString and TryParse and IndexOfAny:

//1. Checks for Length of at least 3 characters
//2. Tries parsing to a number
//3. Check to make sure a successful parse is a number and doesn't contain {, . + -}. 
//Don't want false positives of things like +10 or -99 etc.
public static bool CheckUsername(string user)
{
    return user?.Length >= 3 && 
           Int32.TryParse(user.Substring(user.Length - 3), out _) && 
           user.Substring(user.Length - 3).IndexOfAny(new char[] { ',', '+', '-', '.' }) == -1;
}

edited Sep 30 '20 at 21:37

answered Sep 30 '20 at 20:51

Ryan Wilson

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@MikeJ True. DIdn't think about the negative posibility. – Ryan Wilson Sep 30 '20 at 20:59
Sorry missed that - didn't see it scrolled off to the right. Now I see your point, and yeah seems reasonable. Seems like more work than just checking the string however user[^1] gives you the last digit for example. – MikeJ Sep 30 '20 at 21:14
1

@MikeJ No worries. You made a good point. And your answer may be better. This is just another possibility. – Ryan Wilson Sep 30 '20 at 21:15
Using tryparse and the value was cool, will clean up the other comments. – MikeJ Sep 30 '20 at 21:20
@MikeJ I updated my answer. `001` failed the test. So I checked for `{, . - +}` – Ryan Wilson Sep 30 '20 at 21:29
Good point about the leading zeros, something else I didn't think of. I think you're right, and I think I'm back to thinking I was right, you should just check the 3 digits and not use TryParse. – MikeJ Sep 30 '20 at 21:30
1

@MikeJ Probably. :P But it's different than other proposed solutions, so I think I'll leave it. – Ryan Wilson Sep 30 '20 at 21:31

score 0 · Answer 5 · answered Sep 30 '20 at 21:29

There's also the old-school method for doing this: Check the length, then check the last three characters using a loop:

public static bool Last3AreDigits(string name)
{
    if (name == null || name.Length < 3) return false;

    for (int i = name.Length - 1; i > name.Length - 4; i--)
    {
        if (!char.IsDigit(name[i])) return false;
    }

    return true;
}

How to check if a username has 3 digits

5 Answers5

Original answer (checking the last 3 characters are digits)