using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
        string pattern = @"(?i)\bInvoice[ _:.]+(?<anr>[A-Z/-]*[0-9][A-Z0-9/-]*)(?!\S)";
        string input = @"Payer:_xxxxxxx
Waybill Invoice_5IN280625-3/4 Nimetus_EAN  
Invoice issuer: John
Selle Invoice tasumisel palume m lisada Kesolev Invoice on poolte
vahel Invoice mittetähtaegsel tasumisel on müüjal õigus nõuda viivist 0,1% tasumata summalt päevas.
Kuni Invoice  Second280625-3/4 sumiseni k";
    
        Match m = Regex.Match(input, pattern);
        if (m.Success) {
            Console.WriteLine(m.Groups["anr"]);
        }
	}
}