using System;
using System.Linq;
using System.Collections.Generic;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
        var rx = new Regex(@"^((?!(^|\D)\d\d(\D|$)).)*$");
 
        var textArr = new List<string>()
        {
            "Это выведется 1",
            "15 А это нет",
            "И это 234 выведется",
            "И это даже 2834 выведется",
            "Это 1 выведется 12 хотя не должно 123.",
        };
 
        var filteredArr = textArr.Where(s => rx.IsMatch(s));
 
        Console.WriteLine(String.Join("\n", filteredArr));
	}
}

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