Regular Expression C# Random Letter from specific List of Letters

Question

I want the string (length 8) format to be one letter from a specific letter list A, B, C (neither of these letters should be repeated) the letters have a random position and the rest are numbers (0-9). For example:

1234A567  - Valid
277897C0  - Valid
A100299B  - Not valid
12C3879C  - Not valid

I tried something like this: (\d){7}(A|C|E|F|G|H|J|K|M|U|Z){1} but this does not work. Help?

Answer 1

The current regular expression you have will only match a string with 7 digits and one letter at the end - however you want the letter to be at any random place in the string - so you have to do something different.

Here's a regular expression that will validate everything you need except the total length of the string (but that can be simply tested using str.Length == 8):

^\d{0,7}[A|C|E|F|G|H|J|K|M|U|Z]{1}\d{0,7}$

Here's a simple demo:

var re = new Regex(@"^\d{0,7}[A|C|E|F|G|H|J|K|M|U|Z]{1}\d{0,7}$");
var strings = new string[] {
    "1234A567", // - Valid
    "277897C0", // - Valid
    "A100299B", // - Not valid
    "12C3879C" // - Not valid
};

foreach(var str in strings)
{
    Console.WriteLine(str + (str.Length == 8 && re.IsMatch(str) ? " - valid" : " - not valid"));    
}

Results:

1234A567 - valid
277897C0 - valid
A100299B - not valid
12C3879C - not valid