The bracketing construct "( ... )" creates capture
  buffers.  To refer to the digit'th buffer use \<digit>
  within the match.  Outside the match use "$" instead of
  "\".  (The \<digit> notation works in certain circum­
  stances outside the match.  See the warning below about \1
  vs $1 for details.)  Referring back to another part of the
  match is called a backreference.