RegEx rules and .xlsx

I was wondering if anyone here could help me... I have a lot of .xlsx files with embedded tags and placeholders such as <b></b> and {1} and I'd like to protect these in Trados Studio so that I don't mess them up. I have tried the "catch-all" string solution suggested by Paul in his blog post (Options>File Types>Microsoft Excel 2007-2013>Embedded content), i.e. <[a-z][a-z0-9]*[^<>]*> </[a-z][a-z0-9]*[^<>]*> and I have also tried adding rules for <b></b> separately, but the view in Studio is exactly the same.
I have no idea what I'm doing wrong here...