Code Golf: Sierpinski’s Triangle

J

46 characters, reading from stdin.

(,.~,~[,.~' '$~#,#)^:(<:".1!:1]3)' /\',:'/__\'

\n always delimits sentences, which made it impossible to fit inside S₃ (only 54 characters to play with). S₄ is a bit big at 162, so I padded it to fit. Serendipitously, /\ is a legal adverb. ☺

               /\
              i=:3
             /\  /\
            %r=:1!:1
           /\      /\
          t=:]    [r+i
         /\  /\  /\  /\
        b=:' /\',:'/__\'
       /\              /\
      i=:1            -".t
     /\  /\          /\  /\
    h=:(' '$        ~#,#),.]
   /\      /\      /\      /\
  s=:(    h^:1    ,d=:    ,.~)
 /\  /\  /\  /\  /\  /\  /\  /\
(,,&(10{a.)"1[s^:(-i)b)(1!:2)(4)