The regular expression:

(?-imsx:^.T2|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):
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  .                        any character except \n
----------------------------------------------------------------------
  T2                       'T2'
----------------------------------------------------------------------
 |                        OR
----------------------------------------------------------------------
  S                        'S'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------