/
  (?:         # non capture group start
  ftp:\/{2}   # ftp://
  )?          # end optional non-capture group
  (?:         # non capture group start
  .*?\/       # anything non-greedy, followed by a /
   ){5}       # end non-capture group, match five times
  (.*?)       # capture everything, non-greedy
  \/          # until the very next fwd slash
/x
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Note that you can stringify that into a single line and put it back into the variable. The x modifier allows you to have whitespace in the regex for clarity, and adding comments.