using System;
using System.IO;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		var s = "5123456";
        var matches = Regex.Matches(s, @"\b(5\d{6}|1\d{5})\b");
        foreach (Match m in matches)
        		Console.WriteLine(m.Value);
        
	}
}