using System.Text;

using System.Globalization;

using System;

using System.Text.RegularExpressions;

​

public static class Program

{

    public static void Main(string[] args)

    {

        var text = "PB 54521 PT 012365";

        var result = text.ExtractNumber(5);

        Console.WriteLine(result);

      }

    public static int ExtractNumber(this string text, int length)

        {

            var result = Regex.Match(text, "\b[0-9]{5}\b");

            return result.Success ? int.Parse(result.Groups[0].Value) : 0;

        }

}