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:        and the current character are word, or are not.
  * `\A` matches at the subject string begin.
  * `\z` matches at the subject string end.
  * `\K` matches anything, and reset the start position of the matching
         text to the current position.
  
  More complex assertions can be expressed with lookarounds:
  
  
negative.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 as few characters
  as possible.
  
  Zero width assertions
  ---------------------
  