Anchors                 The start or end of a string.
Backreferences          Backreferences
CharacterClasses        Class Constants
ClassGroups             Character classes
Concatenation           Combine strings together
ReplacementCase         Force the case of replacement values
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
