using System;
using System.Linq;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		var regex = new Regex(@"(?<=headerb)[\r\n]*(?:aa(?<number>\d+)aaa[\r\n]*)+(?=headerc)", RegexOptions.Singleline);
        var match = regex.Match(_input);
        if (match.Success)
        {
            foreach (var number in match.Groups["number"].Captures.Cast<Capture>())
            {
                Console.WriteLine(number);
            }
        }
	}
	
	private static readonly string _input = @"headera
aa1aaa
aa2aaa
aa3aaa

headerb
aa4aaa
aa5aaa
aa6aaa

headerc
aa7aaa
aa8aaa
aa9aaa";
}