using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		var l = new List<string> {"via caporale degli zuavi 278a , 78329","autostrada a1 km - 47"};
		foreach (var t in l) 
		{
			var rx = @"^(?!.*(?<!\p{L})km\b)(?:.*\D)?(\d{1,4})(?=\p{L}?\b)";
			var match = Regex.Match(t, rx, RegexOptions.ECMAScript)?.Groups[1].Value;
			if (!string.IsNullOrEmpty(match))
			{
				Console.WriteLine($"There is a match in '{t}': {match}");
			} 
			else
			{
				Console.WriteLine($"There is no match in '{t}'.");
			}
		}
	}
}