Simple regex based string templating

Program.cs

class Program
{
    static void Main(string[] args)
    {

        Template t = new Template("My {pet} {name} has fleas");
        string[] keys = {"dog", "name"};
        IEnumerable<string> missing, unrecognized;

        bool isValid = t.Validate(keys, out missing, out unrecognized);

        Console.WriteLine("Is valid: " + isValid);
            foreach (string key in unrecognized)
            {
                Console.WriteLine("Unrecognized: " + key);
            }
            foreach (string key in missing)
            {
                Console.WriteLine("Missing: " + key);
            }

        var templateArgs = new Dictionary<string, object> {{"pet", "dog"}, {"name", "spot"}};
        Console.WriteLine(t.Render(templateArgs));
        Console.ReadLine();

    }
}

Template.cs

public class Template
{

    public readonly string Body;
    private HashSet<string> _keys;
    public const string KeyExpression = @"{(?<key>[^}]+)}";

    public Template(string body)
    {
        Body = body;
        //Extract all the keys to a hashset
        _keys = new HashSet<string>();
        Regex.Replace(Body, KeyExpression, k => { 
            _keys.Add(k.Groups["key"].Value);
            return "";
        });

    }

    public bool Validate(IEnumerable<string> keys, out IEnumerable<string> missing, out IEnumerable<string> unrecognized)
    {
        keys = keys.ToArray();
        unrecognized = _keys.Except(keys).ToArray();
        missing = keys.Except(_keys).ToArray();
        return unrecognized.Count() + missing.Count() == 0;
    }

    public string Render(IDictionary<string,object> args)
    {
        return Regex.Replace(Body, KeyExpression, m => Replace(m, args));
    }        

    private string Replace(Match match, IDictionary<string,object> args)
    {
        string key = match.Groups["key"].Value;
        string result = "{" + key + "}";
        if (args.ContainsKey(key)) result = args[key].ToString();
        return result;
    }


    public static Template LoadFromFile(string path)
    {
        string body = File.ReadAllText(path);
        return new Template(body);
    }
}