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
  3. In string index to contain and index or range of indices into a string:
  4. The [] symbol denotes the empty set: