c# matching multiple word boundaries in a single string - regex OR pattern

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


class MainClass {
public static void Main (string[] args) {
var input = "my bull dog is missing cat";
var topics = new List<string>();
topics.Add("cat");
topics.Add("bull dog");
string pattern = string.Join("|", topics.Select(x => @"\b" + @x + @"\b"));

var regex = new Regex(pattern, RegexOptions.IgnoreCase);

var m = regex.Matches(input);
foreach (Match match in m)
{
int i = match.Index;
Console.WriteLine(i);
}
}
}