Anchors                 The start or end of a string.
Backreferences          Backreferences
CharacterClasses        Class Constants
ClassGroups             Character classes
Concatenation           Combine strings together
SpecialCharacters       Special characters
WordBoundaries          Word boundaries
as.regex                Convert or test for regex objects
capture                 Capture a token, or not
char_class              A range or char_class of characters
escape_special          Escape special characters
format.regex            Print or format regex objects
literal                 Treat part of a regular expression literally
lookahead               Lookaround
modify_mode             Apply mode modifiers
or                      Alternation
recursive               Make the regular expression recursive.
regex                   Create a regex
repeated                Repeat values
