using System;
using System.Text.RegularExpressions;
 
public class Example
{
    public static void Main()
    {
        string pattern = @"(?<!\S)(?:x+(?:-x+)*|\d+(?:-\d+)*)$";
        string input = @"Isaiah Kinney 06/2021 111111
Darius Knox 10/2020 111-334-555
Leo Wiley 07/2020 122-333
Stone Walls 11/2020 2112333
John Stone 12/2021 xxx-xx-xxx";
        RegexOptions options = RegexOptions.Multiline;
 
        foreach (Match m in Regex.Matches(input, pattern, options))
        {
            Console.WriteLine("'{0}' found at index {1}.", m.Value, m.Index);
        }
    }
}

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