anchor.diff:        character, or at the subject begin (except if specified that the
         subject begin is not a start of line).
   * `$` matches at the end of a line, that is just before a new line, or
  -      at the subject end (except if specified that the subject end
  +      at the subject end (except if specified that the subject's end
         is not an end of line).
   * `\b` matches at a word boundary, when one of the previous character
          and current character is a word character, and the other is not.
  
positive.code: Kakoune regex syntax is based on the ECMAScript syntax, as defined by the
  ECMA-262 standard.
  
  Kakoune's regex always run on unicode codepoint sequences, not on bytes.
  
  Literals
  --------
  
negative.code: range are treated as literal `-`, so `[A-Z-+]` matches all upper case
  characters, the `-` character, and the `+` character.
  
  Supported character class escapes are:
  
  * `\d` which matches all digits.
  * `\w` which matches all word characters.
  * `\s` which matches all whitespace characters.
  * `\h` which matches all horizontal whitespace characters.
  
  Using an upper case letter instead of a lower case one will negate
  the character class, meaning for example that `\D` will match every
  non-digit character.
  
  Character class escapes can be used outside of a character class, `\d`
  is equivalent to `[\d]`.
  
  Any character
  