using System;
using System.Text.RegularExpressions;
 
public class Test {
    public static void Main() {
        string pattern = "<from_id>(.*?)</from_id>";
        string input = "<?xml version=\"1.0\" encoding=\"utf-8\"?><response list=\"true\"><count>2802</count><post><id>4210</id><from_id>2176594</from_id><to_id>-11423648</to_id><date>1365088358</date><text>dsadsad #ADMIN</text>";
        Match match = Regex.Match(input, pattern);
        if (match.Success){
            System.Console.WriteLine(match.Groups[1].Value);
        }
    }
}

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