using System;
using System.Text.RegularExpressions;
using System.Text;

public class Test
{
    public static void Main()
	{
        string s = "NEW ITEM:1_BELT:3_JEANS:1_BELT:1_SUIT 3 PCS:1_SHOES:1";
        var output = Regex.Replace(s, @"[^0-9]+", "");
        StringBuilder sb = new StringBuilder();
        foreach (var i in output)
        {
            sb.Append(":" + i);
        }
        Console.WriteLine(sb); // :1:3:1:1:3:1:1
	}
}