using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        var text = @"-A line

-Another line

Some other text

-Enter the dragon";

        var rx = new Regex("^-([^ ])", RegexOptions.Multiline);

        text = rx.Replace(text, "- $1");

        Console.WriteLine("/" + text + "/");

    }

}