%!PS

/center {
  % move cursor position to the left by half the width
  dup stringwidth pop 2 div neg 0 rmoveto
} bind def

/Helvetica findfont 12 scalefont setfont

% draw "ABC" left-aligned (as normal) at (100,100)
100 100 moveto (ABC) show

% draw "ABC" horizontally centered around (100,100)
100 100 moveto (ABC) center show