using System;
using System.Text.RegularExpressions;
class RegexSubstitution
{
public static void Main()
{
string text = "124 street";
if ( !Regex.Match( text, @"^[0-9]+\s+([a-zA-Z]+|[a-zA-Z]+\s[a-zA-Z]+)$" ).Success ) {
Console.WriteLine( "Invalid address");
}
}
}