using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var input = "18 / 49 St Clair Avenue St Clair SA";

        var match = Regex.Match(input, @"(?:.\S+?)(\s?)\/(\s?).*");

        while(match.Success)

        {

            if(match.Groups[1].Length > 0){

                input = input.Remove(match.Groups[1].Index, match.Groups[1].Length);

                match = Regex.Match(input, @"(?:.\S+?)(\s?)\/(\s?).*");

                continue;

            }

​

            if(match.Groups[2].Length > 0){

                input = input.Remove(match.Groups[2].Index, match.Groups[2].Length);

                match = Regex.Match(input, @"(?:.\S+?)(\s?)\/(\s?).*");

                continue;

            }

​

            break;

        }

        Console.WriteLine(input);

    }

}