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);
}
}
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