Regex with 3 digits or 4 alphanumeric characters c#-CodePudding

I need to create a regex to validate if a string has a maximum of 3 digits or a maximum of 4 alphanumeric characters, for example those are valid:

123, 000, abc1, a1bc, 1abc, 123a

This ones shouldn't be valid:

1234, 123mx, abcd0

This is my code:

using System;
using System.Text.RegularExpressions;

public class Program
{
    public static void Main()
    {
        var text = "abcd";
        var newRegx = @"(^[0-9]{1,3}$)|(^[a-zA-Z0-9]{1,4}$)";    
        Regex rgx = new Regex(newRegx);
        
        Console.WriteLine ("regex: "  newRegx);
        Console.WriteLine ("\n'" (text="0000") "' matches? "  rgx.IsMatch(text) );
        Console.WriteLine ("\n'" (text="01mk") "' matches? "  rgx.IsMatch(text) );
        Console.WriteLine ("\n'" (text="k") "' matches? "  rgx.IsMatch(text) );
        Console.WriteLine ("\n'" (text="987io") "' matches? "  rgx.IsMatch(text) );
        Console.WriteLine ("\n'" (text="ff123") "' matches? "  rgx.IsMatch(text) );
        Console.WriteLine ("\n'" (text="12") "' matches? "  rgx.IsMatch(text) );
        Console.WriteLine ("\n'" (text="102n") "' matches? "  rgx.IsMatch(text) );
        Console.WriteLine ("\n'" (text="1230") "' matches? "  rgx.IsMatch(text) );
    }
}

It produces this:

regex: (^[0-9]{1,3}$)|(^[a-zA-Z0-9]{1,4}$)
'0000' matches? True
'01mk' matches? True
'k' matches? True
'987io' matches? False
'ff123' matches? False
'12' matches? True
'102n' matches? True
'1234' matches? True

You can run it here: https://dotnetfiddle.net/vWyzD0

The problem is with last example (1234), it shouldn't be valid but my regex is assuming that it is an alphanumeric string and not a number, how can fix my regex?

CodePudding user response：

add negative lookahead (?!\d{4}) before the second part of or statement (^[0-9]{1,3}$)|(^(?!\d{4})[a-zA-Z0-9]{1,4}$) so that 4 digits aren't accepted

https://dotnetfiddle.net/CbGSlG