using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var regex = new Regex(@"^(?:(?<RName>\n[\w]*)\s+)*$", RegexOptions.Compiled);

        var input = "R-abcabc \nR11 abc abc abc \nR20/21/22 abc abc abc \nR34 abc abc abc";

        var matches = regex.Matches(input);

        foreach(Match match in matches) {

             Console.WriteLine(match.Groups["RName"].Value);

        }

    }

}