my $regex = qr{
\A                  # anchor at start for efficiency
  (?=               # positive lookahead for
    (?:
      .*            # anything
      r             # with an 'r' after it
    ){1}            # at least once
  )
  (?=               # positive lookahead for
    (?:.*t){1}      # a 't'
  )
  (?=               # positive lookahead for
    (?:.*o){2}      # for two 'o's
  )
}six;               # ignore case