Watched words of type 'replace' or 'link' replaced the text inside
mentions or hashtags too, which broke these. These types of watched
words must skip any match that has an @ or # before it.
The generated regular expressions did not contain \b which matched
every text that contained the word, even if it was only a substring of
a word.
For example, if "art" was a watched word a post containing word
"artist" matched.
It was not clear that replace watched words can be used to replace text
with URLs. This introduces a new watched word type that makes it easier
to understand.
