The “/ /” is not part of the regular expression; it’s only necessary in some programming languages to signify that the text in between is a regular expression. With SpamSieve, you enter only the regular expression itself. Likewise, you can leave off the “i” because SpamSieve regexes are case-insensitive by default.
So in this case you would enter just:
However, for this example a regex is not required; you could also use Contains instead of Matches Regex.