brackets

The square brackets [ and ] have four functions in CQL
  1. In a textual regular expression, to denote a class of characters:
    "mate in 29"~~[a-z ]+[0-9]+
  2. In a [ and ] in piece designators, to denote a class of pieces or squares
    	K[a1,b3]
    	Q[a-h1-5]
    	[QRBN]a2 attacks [qrbn][a1-8]
  3. In string index to contain and index or range of indices into a string:
    	"stalemate"[3]
    	"mate"[1:5]
  4. The [] symbol denotes the empty set:
    	x=[]