The regular expression:

(?-imsx:This (is)- my first string)

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):
----------------------------------------------------------------------
  This                     'This '
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    is                       'is'
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  - my first string        '- my first string'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------