The regular expression:

(?-imsx:[A-Z]*.pdf)

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):
----------------------------------------------------------------------
  [A-Z]*                   any character of: 'A' to 'Z' (0 or more
                           times (matching the most amount possible))
----------------------------------------------------------------------
  .                        any character except \n
----------------------------------------------------------------------
  pdf                      'pdf'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------