using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var subject = "Loan Number [1234567891] Borrower: [Alex]";

        string loanNumber = string.Empty;

        var groups = new Regex(@"^(?<Start>[^\[]*[\[ \s]*)(?<LoanNumber>\d{10})(?<End>[\s\]]*[a-zA-Z0-9\s]*)$", RegexOptions.ExplicitCapture, TimeSpan.FromSeconds(1)).Match(subject).Groups;

        loanNumber = groups["LoanNumber"].Value;

        Console.WriteLine(loanNumber);

    }

}