using System;
using System.Collections.Generic;
using System.Text.RegularExpressions;
using System.Linq;
public class Test
{
	public static void Main()
	{
		var s = "- - - some text followed by more - - followed by more - - ";
		var res = Regex.Replace(s, @"^\W+", m => m.Value.Replace("-", "~"));
		Console.WriteLine(res);
		// => ~ ~ ~ some text followed by more - - followed by more - - 
		Console.WriteLine(Regex.Replace(s, @"\G([^\w-]*)-", "$1~"));
		// => ~ ~ ~ some text followed by more - - followed by more - - 
		Console.WriteLine(Regex.Replace(s, @"(?<=^\W*)-", "~"));
		// => ~ ~ ~ some text followed by more - - followed by more - - 
	}
}