The regular expression:

(?i-msx:<SomeTag>(.*?)</SomeTag>)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?i-msx:                 group, but do not capture (case-insensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  <SomeTag>                '<SomeTag>'
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    .*?                      any character except \n (0 or more times
                             (matching the least amount possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  </SomeTag>               '</SomeTag>'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------