Checking if a string contains a specific word C#

Question

I am checking these strings to see if they contain the word "hi" and returning true if they do. otherwise i am returning false. the string "high up should return false but is returning true. How can i fix this?

    public static bool StartHi(string str)
    {            
        if (Regex.IsMatch(str, "hi"))
        {
            return true;
        }
        else
            return false;
    }

    static void Main(string[] args)
    {
        StartHi("hi there");    // -> true
        StartHi("hi");          // -> true
        StartHi("high up");     // -> false (returns true when i run)
    }

Solution 1

Try specifying word boundaries (\b):

if(Regex.IsMatch(str, @"\bhi\b"))

Solution 2

private static bool CheckIfExists(string sourceText, string textToCheck)
    {
        return sourceText.Split(' ').Contains(textToCheck);
    }

Comments

[DanKodi]

So, Isn't a word a substring?