Is there a method in c# to check if a string is a valid hexadecimal number?

Question

I wanted to add a manual hexadecimal input for a C# repository of mine, I had no way of verifying if the user's input was a legitimate ARGB hexadecimal value the user had entered into the textbox or if it was a garbage hexadecimal number. Does anyone have a potential solution to this?

Note that the input doesn't just needs to be a hex number, it also needs to be a valid ARGB value. So it must be exactly 8 chars long, unless you want to support a default value for the A (FFFFFF = opaque white) and/or "single digit bytes" (FFF = white) — Hans Keﬆing, Jul 01 '20 at 06:43
What if the user was to input a non valid ARGB value such as #GGGGGG? — Virtualization, Jul 01 '20 at 07:21
Since it's user input validation, it doesn't have to be performant. How this string is used afterwards? I guess you have a method which takes such string as an input. You can simply run this method inside `try/catch` to see if it works. — Sinatr, Jul 01 '20 at 07:38
Does this answer your question? [How do I get the color from a hexadecimal color code using .NET?](https://stackoverflow.com/questions/2109756/how-do-i-get-the-color-from-a-hexadecimal-color-code-using-net) — bvpb, Jul 01 '20 at 22:07

Zze · Answer 1 · 2020-07-01T21:58:59.647

3

You can just use regex:

string pattern = @"^0[xX][0-9a-f]{8}$";
string input = "0x1a2b3C";
Match m = Regex.Match(input, pattern, RegexOptions.IgnoreCase);
if (m.Success)
{...}

edited Jul 01 '20 at 21:58

answered Jul 01 '20 at 05:37

Zze

18,229
13
85
118

8

1. the `0x` prefix is a c# thing, I doubt the users will use that. 2. If you're already going with `RegexOptions.IgnoreCase`, there's no point of using `a-fA-F` in the pattern as well. 3. I think the `\g` here is not needed. Instead, you should use `^` at the beginning and `$` at the end, to indicate that the entire string should match the regular expression. 4. argb values are 8 digits long - so instead of `+` you should use `{8}`. – Zohar Peled Jul 01 '20 at 05:42

score -1 · Answer 2 · answered Jul 01 '20 at 21:58

So, I did a very inefficient method, I started off by making a pastebin with every single valid ARGB hex code, I then wrote the following:

WebClient Checker = new WebClient();
string List = Checker.Downloadstring("https://pastebin.com/raw/link");//link would be the pastebin
string Input = this.TextBox.Text;
if (List.Contains(Input))
{
   //submit write file code here
}
else
{
  System.Windows.Messagebox.Show("The Input Was Not Valid! Please Input A Valid ARGB Code!", "Error!");
}

This method ended up working for me. It isn't recommended but does the job right.

Is there a method in c# to check if a string is a valid hexadecimal number?

2 Answers2