using System;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
		string pattern = @"(?x)Required\sDate
(?<Line>__
(?<PartNo>[a-zA-Z0-9-]*\s[0-9.]+)
)*";
		string str = @"__Required Date__40X0343 1.00__C734X77G 2.00__Net Order:__Sales Tax:__Freight:__Order Total:__0.00 __0.00 __5,328.50 __5,328.50 __or by fax";		
		Regex regex = new Regex(pattern);
		MatchCollection matchColl = regex.Matches(str);
		if (matchColl != null) 
		    foreach (Match match in matchColl) 
		        foreach (Capture c in match.Groups["PartNo"].Captures) 
		            Console.WriteLine(c.Value);
	}
}

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