anchor.diff:  |*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)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)
  
positive.code: |`^`              |at beginning of text or line (`m`=true)
  |`$`              |at end of text (like `\z` not `\Z`) or line (`m`=true)
  |[[escape-sequences]]*Escape Sequences* 1+|
  |`\a`             |bell (≡ `\007`)
  |`\f`             |form feed (≡ `\014`)
  |`\t`             |horizontal tab (≡ `\011`)
  |`\n`             |newline (≡ `\012`)
  |`\r`             |carriage return (≡ `\015`)
  |`\v`             |vertical tab character (≡ `\013`)
  |`\*`             |literal `*`, for any punctuation character `*`
  |`\123`           |octal character code (up to three digits)
  |`\x7F`           |hex character code (exactly two digits)
  |`\x{10FFFF}`     |hex character code
  |[[ascii-character-classes]]*ASCII Character Classes* 1+|
  |`[[:alnum:]]`    |alphanumeric (≡ `[0-9A-Za-z]`)
  |`[[:alpha:]]`    |alphabetic (≡ `[A-Za-z]`)
  |`[[:ascii:]]`    |ASCII (≡ `\x00-\x7F]`)
  |`[[:blank:]]`    |blank (≡ `[\t ]`)
  |`[[:cntrl:]]`    |control (≡ `[\x00-\x1F\x7F]`)
  |`[[:digit:]]`    |digits (≡ `[0-9]`)
  |`[[:graph:]]`    |graphical (≡ `[!-~] == [A-Za-z0-9!"#$%&'()*+,\-./:;<=>?@[\\\\]^_`` `{\|}~]`)
  |`[[:lower:]]`    |lower case (≡ `[a-z]`)
  |`[[:print:]]`    |printable (≡ `[ -~] == [ [:graph:]]`)
  |`[[:punct:]]`    |punctuation (≡ ++[!-/:-@[-`{-~]++)
  |`[[:space:]]`    |whitespace (≡ `[\t\n\v\f\r ]`)
  |`[[:upper:]]`    |upper case (≡ `[A-Z]`)
  |`[[:word:]]`     |word characters (≡ `[0-9A-Za-z_]`)
  |`[[:xdigit:]]`   |hex digit (≡ `[0-9A-Fa-f]`)
  |=======================
  
  [[unsupported-regexp-patterns]]
  
negative.code: |`\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 (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_]`)
  |=======================
  
  
  