This is the second time this week where I have had to ask myself "how did I not know about this?"
There is a regex character to identify word boundaries: \b This is a zero length match, similar to the caret and dollar sign. It finds the boundaries between words, allowing you to search for a whole word match.
Below is a sample extension method that uses this to replace words in a string.
public static class StringExtensions
private static readonly Regex WordRegex = new Regex(@"\b\w+\b", RegexOptions.Compiled);
public static string ReplaceWords(
this string input,
StringComparison comparison = StringComparison.InvariantCulture)
return WordRegex.Replace(input, m => m.Value.Equals(find, comparison)