using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
		var s = "abc,def,2,100,xyz!,:))))";
       	Console.WriteLine(Regex.Replace(s, @"(\d),(\d)", "$1$2"));   // Does not handle 1,2,3,4 cases
       	Console.WriteLine(Regex.Replace(s, @"(\d),(?=\d)", "$1"));   // Handles consecutive matches with capturing group+backreference/lookahead
       	Console.WriteLine(Regex.Replace(s, @"(?<=\d),(?=\d)", ""));  // Handles consecutive matches with lookbehind/lookahead
       	Console.WriteLine(Regex.Replace(s, @",(?<=\d,)(?=\d)", "")); // Also handles all cases, the most efficient way
       	Console.WriteLine(Regex.Replace(s, @"\d,\d", m => m.Value.Replace(",",string.Empty))); // Callback method
	}
}

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