using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
    	var pattern = "a[^b]+b|^[^b]+b";
    	var result = Regex.Match("123412a123b", pattern, RegexOptions.RightToLeft);
        if (result.Success)
        	Console.WriteLine(result.Value);
	}
}