using System;
using System.Text.RegularExpressions;
public class Test
{
    public static void Main()
    {   
        string msg = @"""id"":""456138988365628440_103920"",""user""657852231654""";
        var reg = new Regex(@"""id"":""([0-9_]*)"",""user""", RegexOptions.IgnoreCase);
        var results = reg.Matches(msg);
        foreach (Match match in results)
        {
            Console.WriteLine(match.Groups[1].Value);
        }
    }
}

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