using System;
using System.Linq;
using System.Text.RegularExpressions;
public class Test
{
	public static void Main()
	{
		string input = "***XX*123456789~N3*123 E. Fake St. Apt# 456~N4*Beverly Hills*CA*902122405~REF*EI*902122405~HL*1*1*50*0~SBR*P*18*******MA~NM1*IL*1*Tom*Thompson*T***MI*123456789A~N3*456 W. False Ave.*Apt. #6B~N4*Beverly Hills*CA*90210~DMG*";
		string matchPattern = @"(~N3\*)(.*?)(~N4\*)";
		string replacePattern = "[^0-9a-zA-Z ]";
 
		var res = Regex.Replace(input, matchPattern, m =>
				string.Format("{0}{1}{2}", 
				m.Groups[1].Value, 
				Regex.Replace(m.Groups[2].Value, replacePattern, " "),
				m.Groups[3].Value));
		Console.Write(res);
	}
}

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