using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		var text = "Text  4000751111115425";
    	var result = Regex.Match(text, @"(?<!\d)\d{6}(\d+)(?=\d{4}(?!\d))")?.Groups[1].Value;
        Console.WriteLine($"Result: '{result}'");
	}
}