Multi-file regular expression find & replace functionality, with nameable saveable expressions
That's the killer-app feature for me. I could actually say that I think it's BBEdit that gave me my first glimpse of the power of Perl.
Also, if you're a Perl & BBEdit user and haven't already, check out the BBEdit Help -> Grep reference, it is a very useful reference for all the PCRE stuff, especially useful for things in the Advanced Grep Topics section like zero width negative lookbehind assertions (which if you're like me you can never remember what sequence of line noise they might correspond to).
BBEdit is a very powerful text editor, but where it really out shines other text editors is its attention to detail (like handling a file being renamed underneath it and updating as the file changes, and such) and the way it handles multiple files (I like the Window palette, but now also with multiple documents per window), especially for things like multifile find & replace
Enjoy, Peter.
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