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