anchor.diff:  |`\C`              |match a single byte even in UTF-8 mode
   |`\Q...\E`         |literal text `...` even if `...` has punctuation
   |*Unsupported Perl Character Classes*  1+|
  -|`\d`              |digits (≡ `[0-9]`)
  -|`\D`              |not digits (≡ `[^0-9]`)
  -|`\s`              |whitespace (≡ `[\t\n\f\r ]`)
  -|`\S`              |not whitespace (≡ `[^\t\n\f\r ]`)
  -|`\w`              |word characters (≡ `[0-9A-Za-z_]`)
  -|`\W`              |not word characters (≡ `[^0-9A-Za-z_]`)
  +|`\d`              |digits (same as `[0-9]`)
  +|`\D`              |not digits (same as `[^0-9]`)
  +|`\s`              |whitespace (same as `[\t\n\f\r ]`)
  +|`\S`              |not whitespace (same as `[^\t\n\f\r ]`)
  +|`\w`              |word characters (same as `[0-9A-Za-z_]`)
  +|`\W`              |not word characters (same as `[^0-9A-Za-z_]`)
   |=======================
   
   
  
positive.code: |*Unsupported Grouping*      1+|
  |`(?P<name>re)`    |named & numbered capturing group (submatch)
  |`(?:re)`          |non-capturing group
  |`(?i)abc`        |set flags within current group; non-capturing
  |`(?i:re)`         |set flags during re; non-capturing
  |`(?i)PaTTeRN`     |case-insensitive (default false)
  |`(?m)multiline`   |multi-line mode: `^` and `$` match begin/end line in addition to begin/end text (default false)
  |`(?s)pattern.`    |let `.` match `\n` (default false)
  
negative.code: |`^`              |at beginning of text or line (`m`=true)
  |`$`              |at end of text (like `\z` not `\Z`) or line (`m`=true)
  |[[escape-sequences]]*Escape Sequences* 1+|
  |`\a`             |bell (same as `\007`)
  |`\f`             |form feed (same as `\014`)
  |`\t`             |horizontal tab (same as `\011`)
  |`\n`             |newline (same as `\012`)
  |`\r`             |carriage return (same as `\015`)
  |`\v`             |vertical tab character (same as `\013`)
  |`\*`             |literal `*`, for any punctuation character `*`
  |`\123`           |octal character code (up to three digits)
  |`\x7F`           |two-digit hex character code
  |`\x{10FFFF}`     |hex character code
  |[[ascii-character-classes]]*ASCII Character Classes* 1+|
  |`[[:alnum:]]`    |alphanumeric (same as `[0-9A-Za-z]`)
  |`[[:alpha:]]`    |alphabetic (same as `[A-Za-z]`)
  |`[[:ascii:]]`    |ASCII (same as `\x00-\x7F]`)
  |`[[:blank:]]`    |blank (same as `[\t ]`)
  |`[[:cntrl:]]`    |control (same as `[\x00-\x1F\x7F]`)
  |`[[:digit:]]`    |digits (same as `[0-9]`)
  |`[[:graph:]]`    |graphical (same as `[!-~] == [A-Za-z0-9!"#$%&'()*+,\-./:;<=>?@[\\\\]^_`` `{\|}~]`)
  |`[[:lower:]]`    |lower case (same as `[a-z]`)
  |`[[:print:]]`    |printable (same as `[ -~] == [ [:graph:]]`)
  |`[[:punct:]]`    |punctuation (same as ++[!-/:-@[-`{-~]++)
  |`[[:space:]]`    |whitespace (same as `[\t\n\v\f\r ]`)
  |`[[:upper:]]`    |upper case (same as `[A-Z]`)
  |`[[:word:]]`     |word characters (same as `[0-9A-Za-z_]`)
  |`[[:xdigit:]]`   |hex digit (same as `[0-9A-Fa-f]`)
  |=======================
  
  [[unsupported-regexp-patterns]]
  