The regular expression:

(?-imsx:(class|struct)\s+[^:{;]+:[\w<>\s]+{)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    class                    'class'
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    struct                   'struct'
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  \s+                      whitespace (\n, \r, \t, \f, and " ") (1 or
                           more times (matching the most amount
                           possible))
----------------------------------------------------------------------
  [^:{;]+                  any character except: ':', '{', ';' (1 or
                           more times (matching the most amount
                           possible))
----------------------------------------------------------------------
  :                        ':'
----------------------------------------------------------------------
  [\w<>\s]+                any character of: word characters (a-z, A-
                           Z, 0-9, _), '<', '>', whitespace (\n, \r,
                           \t, \f, and " ") (1 or more times
                           (matching the most amount possible))
----------------------------------------------------------------------
  {                        '{'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------