using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		var input = "Lorem 1 ip234sum 22 dolor 3234445 sit amet, adipiscing 1234";
		string result = Regex.Replace(input, @"(?<=\b[\d]{1})[\d]*(?=[\d]{1}\b)", m => new string('*', m.Length));
        Console.WriteLine(result);
	}
}