Regular Expressions for Parsing
Regex with named capture groups and inline mode-setting (should be compatible with Python, PHP, .NET/C#, and the PCRE library):
Regex without named capture groups but with inline mode-setting (should work in Java):
I'm using the first edition of O'Reilly's Regular Expression Pocket Reference, so it's possible that, since it was written, your favourite language gained support for named capture groups.
Make sure you handle malformed entries properly if you're automatically parsing a tag feed from a site like Identi.ca. The announcement of this reference itself contains #fic_rating but doesn't follow the format described by these regexes.