anchor.diff:  Character classes
   -----------------
   
  -The `[` character introduces a character class, which can match multiple
  -characters.
  +The `[` character introduces a character class, matching one character
  +from a set of characters.
   
   A character class contains a list of literals, character ranges,
   and character class escapes surrounded by `[` and `]`.
  
positive.code: 
  By default, quantifiers are *greedy*, which means they will prefer to
  match more characters if possible. Suffixing a quantifier with `?` will
  make it non-greedy, meaning it will prefer to match fewer characters.
  
  Zero width assertions
  ---------------------
  
negative.code: used, the group will be a capturing group, which means the positions from
  the subject strings that matched between `(` and `)` will be recorded.
  
  Capture groups are numbered starting at 1. They are numbered in the order of
  appearance of their `(` in the regex. A special capture group 0 is
  for the whole sequence that matched.
  
  `(?:` introduces a non capturing group, which will not record the
  matching positions.
  
  Alternations
  ------------
  