Why does this happen in regexp isearch?

[Deniz Dogan]

I open cus-edit.el.gz in Emacs and do a regexp isearch for:

^  ;;

(Beginning of line, then two spaces, then two semicolons.)

For some reason that I don't understand, it seems to match "beginning
of line, any number of spaces OR tabs, then two semicolons". When I
search for:

^ \{2\};;

...it finds what I'm looking for. Why is this?

-- 
Deniz Dogan