The regular expression:

(?-imsx:(^USA|BRA|CAN)?(.*?)(XY)?$)

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 (optional
                           (matching the most amount possible)):
----------------------------------------------------------------------
    ^                        the beginning of the string
----------------------------------------------------------------------
    USA                      'USA'
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    BRA                      'BRA'
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    CAN                      'CAN'
----------------------------------------------------------------------
  )?                       end of \1 (NOTE: because you are using a
                           quantifier on this capture, only the LAST
                           repetition of the captured pattern will be
                           stored in \1)
----------------------------------------------------------------------
  (                        group and capture to \2:
----------------------------------------------------------------------
    .*?                      any character except \n (0 or more times
                             (matching the least amount possible))
----------------------------------------------------------------------
  )                        end of \2
----------------------------------------------------------------------
  (                        group and capture to \3 (optional
                           (matching the most amount possible)):
----------------------------------------------------------------------
    XY                       'XY'
----------------------------------------------------------------------
  )?                       end of \3 (NOTE: because you are using a
                           quantifier on this capture, only the LAST
                           repetition of the captured pattern will be
                           stored in \3)
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------