using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
		var inputList = new List<string>() { "bbb aaa.a1 bbb", "ccc aaa.a2 ccc" };
		var result = inputList
			.Select(i => Regex.Match(i, @"\baaa\.\S+")?.Value)
			.Where(x => !string.IsNullOrEmpty(x))
			.ToList();
        foreach (var s in result)
        	Console.WriteLine(s);
	}
}

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