using System;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
		string pattern = @"(?i).*Invoice[\s_]?NR\.?[\s_:.](?<arvenumber>.*?)[\s_]";
        string input = @"...
Some nr_11687767_ other 101308591
Invoice Nr.
M230714_some text
Kirjeldus
...
Sometimes it is terminated by newline
 
...
This nr_11687767_KMKR_EE101308591
Invoice Nr.
M230714
01.05.2023
Item
...
or by other white space delimiter :
 
...
Some  nr_11687767_ Text
Invoice Nr M230714   Date 01.05.2023
Desc
...";
 
            foreach (Match m in Regex.Matches(input, pattern))
            {
                Console.WriteLine("'{0}' found", m.Groups["arvenumber"].Value);
            }
        }
 
}

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