using System;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
		var str = "Теоретики3 и 2п4423ра_ктики палинд4442рома выделили\n" +
				  "2многочис2ленные4 погран24ичные2 с пал24и_нд4ром4м формы";
 
		foreach (Match match in Regex.Matches(str, @"(?=\w*(?!\d)\w)(?=\w*\d)\w+", RegexOptions.IgnoreCase))
			Console.WriteLine(match.Value);
 
		Console.WriteLine("===");
 
		foreach (Match match in Regex.Matches(str, @"\b(?=\w*(?!\d)\w)(?=\w*\d)(?=((?!_)\w)+\b)\w+", RegexOptions.IgnoreCase))
			Console.WriteLine(match.Value);
	}
}

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