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: 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
  ------------
  