regexp -nocase {<%@\s*page\s*((?:(?!%>).)*)%>} $string \
          wholeMatch interestingContent

How does it work? Simple. Firstly, none of <%@> are special chars to RE
by themselves. Secondly, the first part (up to the second \s*) matches
the start of the interesting chunk from your string, and there's a
literal %> at the end. Thirdly, there's a bunch of real RE magic in the
middle:
   ((?:(?!%>).)*)

Looking at that in detail, we see this in expanded form

   (              # Stash this sub-RE in a variable
    (?:           # Pure grouping of zero-width assertion and matcher
      (?! %> ) .  # any character, as long as it isn't the start of %>
    ) *           # and as many of them as possible
   )              # That's All Folks!

The key to doing sane parsing is often the negative lookahead assertion.
It makes life *so* much easier!