# <!-- rdoc-file=re.c -->
# A [regular expression](https://en.wikipedia.org/wiki/Regular_expression) (also
# called a *regexp*) is a *match pattern* (also simply called a *pattern*).
#
# A common notation for a regexp uses enclosing slash characters:
#
#     /foo/
#
# A regexp may be applied to a *target string*; The part of the string (if any)
# that matches the pattern is called a *match*, and may be said *to match*:
#
#     re = /red/
#     re.match?('redirect') # => true   # Match at beginning of target.
#     re.match?('bored')    # => true   # Match at end of target.
#     re.match?('credit')   # => true   # Match within target.
#     re.match?('foo')      # => false  # No match.
#
# ## Regexp Uses
#
# A regexp may be used:
#
# *   To extract substrings based on a given pattern:
#
#         re = /foo/              # => /foo/
#         re.match('food')        # => #<MatchData "foo">
#         re.match('good')        # => nil
#
#     See sections [Method match](rdoc-ref:Regexp@Method+match) and [Operator
#     =~](rdoc-ref:Regexp@Operator+-3D~).
#
# *   To determine whether a string matches a given pattern:
#
#         re.match?('food') # => true
#         re.match?('good') # => false
#
#     See section [Method match?](rdoc-ref:Regexp@Method+match-3F).
#
# *   As an argument for calls to certain methods in other classes and modules;
#     most such methods accept an argument that may be either a string or the
#     (much more powerful) regexp.
#
#     See [Regexp Methods](rdoc-ref:regexp/methods.rdoc).
#
# ## Regexp Objects
#
# A regexp object has:
#
# *   A source; see [Sources](rdoc-ref:Regexp@Sources).
#
# *   Several modes; see [Modes](rdoc-ref:Regexp@Modes).
#
# *   A timeout; see [Timeouts](rdoc-ref:Regexp@Timeouts).
#
# *   An encoding; see [Encodings](rdoc-ref:Regexp@Encodings).
#
# ## Creating a Regexp
#
# A regular expression may be created with:
#
# *   A regexp literal using slash characters (see [Regexp
#     Literals](rdoc-ref:syntax/literals.rdoc@Regexp+Literals)):
#
#         # This is a very common usage.
#         /foo/ # => /foo/
#
# *   A `%r` regexp literal (see [%r: Regexp
#     Literals](rdoc-ref:syntax/literals.rdoc@25r-3A+Regexp+Literals)):
#
#         # Same delimiter character at beginning and end;
#         # useful for avoiding escaping characters
#         %r/name\/value pair/ # => /name\/value pair/
#         %r:name/value pair:  # => /name\/value pair/
#         %r|name/value pair|  # => /name\/value pair/
#
#         # Certain "paired" characters can be delimiters.
#         %r[foo] # => /foo/
#         %r{foo} # => /foo/
#         %r(foo) # => /foo/
#         %r<foo> # => /foo/
#
# *   Method Regexp.new.
#
# ## Method `match`
#
# Each of the methods Regexp#match, String#match, and Symbol#match returns a
# MatchData object if a match was found, `nil` otherwise; each also sets [global
# variables](rdoc-ref:Regexp@Global+Variables):
#
#     'food'.match(/foo/) # => #<MatchData "foo">
#     'food'.match(/bar/) # => nil
#
# ## Operator `=~`
#
# Each of the operators Regexp#=~, String#=~, and Symbol#=~ returns an integer
# offset if a match was found, `nil` otherwise; each also sets [global
# variables](rdoc-ref:Regexp@Global+Variables):
#
#     /bar/ =~ 'foo bar' # => 4
#     'foo bar' =~ /bar/ # => 4
#     /baz/ =~ 'foo bar' # => nil
#
# ## Method `match?`
#
# Each of the methods Regexp#match?, String#match?, and Symbol#match? returns
# `true` if a match was found, `false` otherwise; none sets [global
# variables](rdoc-ref:Regexp@Global+Variables):
#
#     'food'.match?(/foo/) # => true
#     'food'.match?(/bar/) # => false
#
# ## Global Variables
#
# Certain regexp-oriented methods assign values to global variables:
#
# *   `#match`: see [Method match](rdoc-ref:Regexp@Method+match).
# *   `#=~`: see [Operator =~](rdoc-ref:Regexp@Operator+-3D~).
#
# The affected global variables are:
#
# *   `$~`: Returns a MatchData object, or `nil`.
# *   `$&`: Returns the matched part of the string, or `nil`.
# *   `$``: Returns the part of the string to the left of the match, or `nil`.
# *   `$'`: Returns the part of the string to the right of the match, or `nil`.
# *   `$+`: Returns the last group matched, or `nil`.
# *   `$1`, `$2`, etc.: Returns the first, second, etc., matched group, or
#     `nil`. Note that `$0` is quite different; it returns the name of the
#     currently executing program.
#
# Examples:
#
#     # Matched string, but no matched groups.
#     'foo bar bar baz'.match('bar')
#     $~ # => #<MatchData "bar">
#     $& # => "bar"
#     $` # => "foo "
#     $' # => " bar baz"
#     $+ # => nil
#     $1 # => nil
#
#     # Matched groups.
#     /s(\w{2}).*(c)/.match('haystack')
#     $~ # => #<MatchData "stac" 1:"ta" 2:"c">
#     $& # => "stac"
#     $` # => "hay"
#     $' # => "k"
#     $+ # => "c"
#     $1 # => "ta"
#     $2 # => "c"
#     $3 # => nil
#
#     # No match.
#     'foo'.match('bar')
#     $~ # => nil
#     $& # => nil
#     $` # => nil
#     $' # => nil
#     $+ # => nil
#     $1 # => nil
#
# Note that Regexp#match?, String#match?, and Symbol#match? do not set global
# variables.
#
# ## Sources
#
# As seen above, the simplest regexp uses a literal expression as its source:
#
#     re = /foo/              # => /foo/
#     re.match('food')        # => #<MatchData "foo">
#     re.match('good')        # => nil
#
# A rich collection of available *subexpressions* gives the regexp great power
# and flexibility:
#
# *   [Special characters](rdoc-ref:Regexp@Special+Characters)
# *   [Source literals](rdoc-ref:Regexp@Source+Literals)
# *   [Character classes](rdoc-ref:Regexp@Character+Classes)
# *   [Shorthand character classes](rdoc-ref:Regexp@Shorthand+Character+Classes)
# *   [Anchors](rdoc-ref:Regexp@Anchors)
# *   [Alternation](rdoc-ref:Regexp@Alternation)
# *   [Quantifiers](rdoc-ref:Regexp@Quantifiers)
# *   [Groups and captures](rdoc-ref:Regexp@Groups+and+Captures)
# *   [Unicode](rdoc-ref:Regexp@Unicode)
# *   [POSIX Bracket Expressions](rdoc-ref:Regexp@POSIX+Bracket+Expressions)
# *   [Comments](rdoc-ref:Regexp@Comments)
#
# ### Special Characters
#
# Regexp special characters, called *metacharacters*, have special meanings in
# certain contexts; depending on the context, these are sometimes
# metacharacters:
#
#     . ? - + * ^ \ | $ ( ) [ ] { }
#
# To match a metacharacter literally, backslash-escape it:
#
#     # Matches one or more 'o' characters.
#     /o+/.match('foo')  # => #<MatchData "oo">
#     # Would match 'o+'.
#     /o\+/.match('foo') # => nil
#
# To match a backslash literally, backslash-escape it:
#
#     /\./.match('\.')  # => #<MatchData ".">
#     /\\./.match('\.') # => #<MatchData "\\.">
#
# Method Regexp.escape returns an escaped string:
#
#     Regexp.escape('.?-+*^\|$()[]{}')
#     # => "\\.\\?\\-\\+\\*\\^\\\\\\|\\$\\(\\)\\[\\]\\{\\}"
#
# ### Source Literals
#
# The source literal largely behaves like a double-quoted string; see
# [Double-Quoted String
# Literals](rdoc-ref:syntax/literals.rdoc@Double-Quoted+String+Literals).
#
# In particular, a source literal may contain interpolated expressions:
#
#     s = 'foo'         # => "foo"
#     /#{s}/            # => /foo/
#     /#{s.capitalize}/ # => /Foo/
#     /#{2 + 2}/        # => /4/
#
# There are differences between an ordinary string literal and a source literal;
# see [Shorthand Character
# Classes](rdoc-ref:Regexp@Shorthand+Character+Classes).
#
# *   `\s` in an ordinary string literal is equivalent to a space character; in
#     a source literal, it's shorthand for matching a whitespace character.
# *   In an ordinary string literal, these are (needlessly) escaped characters;
#     in a source literal, they are shorthands for various matching characters:
#
#         \w \W \d \D \h \H \S \R
#
# ### Character Classes
#
# A *character class* is delimited by square brackets; it specifies that certain
# characters match at a given point in the target string:
#
#     # This character class will match any vowel.
#     re = /B[aeiou]rd/
#     re.match('Bird') # => #<MatchData "Bird">
#     re.match('Bard') # => #<MatchData "Bard">
#     re.match('Byrd') # => nil
#
# A character class may contain hyphen characters to specify ranges of
# characters:
#
#     # These regexps have the same effect.
#     /[abcdef]/.match('foo') # => #<MatchData "f">
#     /[a-f]/.match('foo')    # => #<MatchData "f">
#     /[a-cd-f]/.match('foo') # => #<MatchData "f">
#
# When the first character of a character class is a caret (`^`), the sense of
# the class is inverted: it matches any character *except* those specified.
#
#     /[^a-eg-z]/.match('f') # => #<MatchData "f">
#
# A character class may contain another character class. By itself this isn't
# useful because `[a-z[0-9]]` describes the same set as `[a-z0-9]`.
#
# However, character classes also support the `&&` operator, which performs set
# intersection on its arguments. The two can be combined as follows:
#
#     /[a-w&&[^c-g]z]/ # ([a-w] AND ([^c-g] OR z))
#
# This is equivalent to:
#
#     /[abh-w]/
#
# ### Shorthand Character Classes
#
# Each of the following metacharacters serves as a shorthand for a character
# class:
#
# *   `/./`: Matches any character except a newline:
#
#         /./.match('foo') # => #<MatchData "f">
#         /./.match("\n")  # => nil
#
# *   `/./m`: Matches any character, including a newline; see [Multiline
#     Mode](rdoc-ref:Regexp@Multiline+Mode):
#
#         /./m.match("\n") # => #<MatchData "\n">
#
# *   `/\w/`: Matches a word character: equivalent to `[a-zA-Z0-9_]`:
#
#         /\w/.match(' foo') # => #<MatchData "f">
#         /\w/.match(' _')   # => #<MatchData "_">
#         /\w/.match(' ')    # => nil
#
# *   `/\W/`: Matches a non-word character: equivalent to `[^a-zA-Z0-9_]`:
#
#         /\W/.match(' ') # => #<MatchData " ">
#         /\W/.match('_') # => nil
#
# *   `/\d/`: Matches a digit character: equivalent to `[0-9]`:
#
#         /\d/.match('THX1138') # => #<MatchData "1">
#         /\d/.match('foo')     # => nil
#
# *   `/\D/`: Matches a non-digit character: equivalent to `[^0-9]`:
#
#         /\D/.match('123Jump!') # => #<MatchData "J">
#         /\D/.match('123')      # => nil
#
# *   `/\h/`: Matches a hexdigit character: equivalent to `[0-9a-fA-F]`:
#
#         /\h/.match('xyz fedcba9876543210') # => #<MatchData "f">
#         /\h/.match('xyz')                  # => nil
#
# *   `/\H/`: Matches a non-hexdigit character: equivalent to `[^0-9a-fA-F]`:
#
#         /\H/.match('fedcba9876543210xyz') # => #<MatchData "x">
#         /\H/.match('fedcba9876543210')    # => nil
#
# *   `/\s/`: Matches a whitespace character: equivalent to `/[ \t\r\n\f\v]/`:
#
#         /\s/.match('foo bar') # => #<MatchData " ">
#         /\s/.match('foo')     # => nil
#
# *   `/\S/`: Matches a non-whitespace character: equivalent to `/[^
#     \t\r\n\f\v]/`:
#
#         /\S/.match(" \t\r\n\f\v foo") # => #<MatchData "f">
#         /\S/.match(" \t\r\n\f\v")     # => nil
#
# *   `/\R/`: Matches a linebreak, platform-independently:
#
#         /\R/.match("\r")     # => #<MatchData "\r">     # Carriage return (CR)
#         /\R/.match("\n")     # => #<MatchData "\n">     # Newline (LF)
#         /\R/.match("\f")     # => #<MatchData "\f">     # Formfeed (FF)
#         /\R/.match("\v")     # => #<MatchData "\v">     # Vertical tab (VT)
#         /\R/.match("\r\n")   # => #<MatchData "\r\n">   # CRLF
#         /\R/.match("\u0085") # => #<MatchData "\u0085"> # Next line (NEL)
#         /\R/.match("\u2028") # => #<MatchData "\u2028"> # Line separator (LSEP)
#         /\R/.match("\u2029") # => #<MatchData "\u2029"> # Paragraph separator (PSEP)
#
# ### Anchors
#
# An anchor is a metasequence that matches a zero-width position between
# characters in the target string.
#
# For a subexpression with no anchor, matching may begin anywhere in the target
# string:
#
#     /real/.match('surrealist') # => #<MatchData "real">
#
# For a subexpression with an anchor, matching must begin at the matched anchor.
#
# #### Boundary Anchors
#
# Each of these anchors matches a boundary:
#
# *   `^`: Matches the beginning of a line:
#
#         /^bar/.match("foo\nbar") # => #<MatchData "bar">
#         /^ar/.match("foo\nbar")  # => nil
#
# *   `$`: Matches the end of a line:
#
#         /bar$/.match("foo\nbar") # => #<MatchData "bar">
#         /ba$/.match("foo\nbar")  # => nil
#
# *   `\A`: Matches the beginning of the string:
#
#         /\Afoo/.match('foo bar')  # => #<MatchData "foo">
#         /\Afoo/.match(' foo bar') # => nil
#
# *   `\Z`: Matches the end of the string; if string ends with a single newline,
#     it matches just before the ending newline:
#
#         /foo\Z/.match('bar foo')     # => #<MatchData "foo">
#         /foo\Z/.match('foo bar')     # => nil
#         /foo\Z/.match("bar foo\n")   # => #<MatchData "foo">
#         /foo\Z/.match("bar foo\n\n") # => nil
#
# *   `\z`: Matches the end of the string:
#
#         /foo\z/.match('bar foo')   # => #<MatchData "foo">
#         /foo\z/.match('foo bar')   # => nil
#         /foo\z/.match("bar foo\n") # => nil
#
# *   `\b`: Matches word boundary when not inside brackets; matches backspace
#     (`"0x08"`) when inside brackets:
#
#         /foo\b/.match('foo bar') # => #<MatchData "foo">
#         /foo\b/.match('foobar')  # => nil
#
# *   `\B`: Matches non-word boundary:
#
#         /foo\B/.match('foobar')  # => #<MatchData "foo">
#         /foo\B/.match('foo bar') # => nil
#
# *   `\G`: Matches first matching position:
#
#     In methods like String#gsub and String#scan, it changes on each iteration.
#     It initially matches the beginning of subject, and in each following
#     iteration it matches where the last match finished.
#
#         "    a b c".gsub(/ /, '_')   # => "____a_b_c"
#         "    a b c".gsub(/\G /, '_') # => "____a b c"
#
#     In methods like Regexp#match and String#match that take an optional
#     offset, it matches where the search begins.
#
#         "hello, world".match(/,/, 3)   # => #<MatchData ",">
#         "hello, world".match(/\G,/, 3) # => nil
#
# #### Lookaround Anchors
#
# Lookahead anchors:
#
# *   `(?=*pat*)`: Positive lookahead assertion: ensures that the following
#     characters match *pat*, but doesn't include those characters in the
#     matched substring.
#
# *   `(?!*pat*)`: Negative lookahead assertion: ensures that the following
#     characters *do not* match *pat*, but doesn't include those characters in
#     the matched substring.
#
# Lookbehind anchors:
#
# *   `(?<=*pat*)`: Positive lookbehind assertion: ensures that the preceding
#     characters match *pat*, but doesn't include those characters in the
#     matched substring.
#
# *   `(?<!*pat*)`: Negative lookbehind assertion: ensures that the preceding
#     characters do not match *pat*, but doesn't include those characters in the
#     matched substring.
#
# The pattern below uses positive lookahead and positive lookbehind to match
# text appearing in **...** tags without including the tags in the match:
#
#     /(?<=<b>)\w+(?=<\/b>)/.match("Fortune favors the <b>bold</b>.")
#     # => #<MatchData "bold">
#
# #### Match-Reset Anchor
#
# *   `\K`: Match reset: the matched content preceding `\K` in the regexp is
#     excluded from the result. For example, the following two regexps are
#     almost equivalent:
#
#         /ab\Kc/.match('abc')    # => #<MatchData "c">
#         /(?<=ab)c/.match('abc') # => #<MatchData "c">
#
#     These match same string and `$&` equals `'c'`, while the matched position
#     is different.
#
#     As are the following two regexps:
#
#         /(a)\K(b)\Kc/
#         /(?<=(?<=(a))(b))c/
#
# ### Alternation
#
# The vertical bar metacharacter (`|`) may be used within parentheses to express
# alternation: two or more subexpressions any of which may match the target
# string.
#
# Two alternatives:
#
#     re = /(a|b)/
#     re.match('foo') # => nil
#     re.match('bar') # => #<MatchData "b" 1:"b">
#
# Four alternatives:
#
#     re = /(a|b|c|d)/
#     re.match('shazam') # => #<MatchData "a" 1:"a">
#     re.match('cold')   # => #<MatchData "c" 1:"c">
#
# Each alternative is a subexpression, and may be composed of other
# subexpressions:
#
#     re = /([a-c]|[x-z])/
#     re.match('bar') # => #<MatchData "b" 1:"b">
#     re.match('ooz') # => #<MatchData "z" 1:"z">
#
# Method Regexp.union provides a convenient way to construct a regexp with
# alternatives.
#
# ### Quantifiers
#
# A simple regexp matches one character:
#
#     /\w/.match('Hello')  # => #<MatchData "H">
#
# An added *quantifier* specifies how many matches are required or allowed:
#
# *   `*` - Matches zero or more times:
#
#         /\w*/.match('')
#         # => #<MatchData "">
#         /\w*/.match('x')
#         # => #<MatchData "x">
#         /\w*/.match('xyz')
#         # => #<MatchData "yz">
#
# *   `+` - Matches one or more times:
#
#         /\w+/.match('')    # => nil
#         /\w+/.match('x')   # => #<MatchData "x">
#         /\w+/.match('xyz') # => #<MatchData "xyz">
#
# *   `?` - Matches zero or one times:
#
#         /\w?/.match('')    # => #<MatchData "">
#         /\w?/.match('x')   # => #<MatchData "x">
#         /\w?/.match('xyz') # => #<MatchData "x">
#
# *   `{`*n*`}` - Matches exactly *n* times:
#
#         /\w{2}/.match('')    # => nil
#         /\w{2}/.match('x')   # => nil
#         /\w{2}/.match('xyz') # => #<MatchData "xy">
#
# *   `{`*min*`,}` - Matches *min* or more times:
#
#         /\w{2,}/.match('')    # => nil
#         /\w{2,}/.match('x')   # => nil
#         /\w{2,}/.match('xy')  # => #<MatchData "xy">
#         /\w{2,}/.match('xyz') # => #<MatchData "xyz">
#
# *   `{,`*max*`}` - Matches *max* or fewer times:
#
#         /\w{,2}/.match('')    # => #<MatchData "">
#         /\w{,2}/.match('x')   # => #<MatchData "x">
#         /\w{,2}/.match('xyz') # => #<MatchData "xy">
#
# *   `{`*min*`,`*max*`}` - Matches at least *min* times and at most *max*
#     times:
#
#         /\w{1,2}/.match('')    # => nil
#         /\w{1,2}/.match('x')   # => #<MatchData "x">
#         /\w{1,2}/.match('xyz') # => #<MatchData "xy">
#
# #### Greedy, Lazy, or Possessive Matching
#
# Quantifier matching may be greedy, lazy, or possessive:
#
# *   In *greedy* matching, as many occurrences as possible are matched while
#     still allowing the overall match to succeed. Greedy quantifiers: `*`, `+`,
#     `?`, `{min, max}` and its variants.
# *   In *lazy* matching, the minimum number of occurrences are matched. Lazy
#     quantifiers: `*?`, `+?`, `??`, `{min, max}?` and its variants.
# *   In *possessive* matching, once a match is found, there is no backtracking;
#     that match is retained, even if it jeopardises the overall match.
#     Possessive quantifiers: `*+`, `++`, `?+`. Note that `{min, max}` and its
#     variants do *not* support possessive matching.
#
# More:
#
# *   About greedy and lazy matching, see [Choosing Minimal or Maximal
#     Repetition](https://doc.lagout.org/programmation/Regular%20Expressions/Reg
#     ular%20Expressions%20Cookbook_%20Detailed%20Solutions%20in%20Eight%20Progr
#     amming%20Languages%20%282nd%20ed.%29%20%5BGoyvaerts%20%26%20Levithan%20201
#     2-09-06%5D.pdf#tutorial-backtrack).
# *   About possessive matching, see [Eliminate Needless
#     Backtracking](https://doc.lagout.org/programmation/Regular%20Expressions/R
#     egular%20Expressions%20Cookbook_%20Detailed%20Solutions%20in%20Eight%20Pro
#     gramming%20Languages%20%282nd%20ed.%29%20%5BGoyvaerts%20%26%20Levithan%202
#     012-09-06%5D.pdf#tutorial-backtrack).
#
# ### Groups and Captures
#
# A simple regexp has (at most) one match:
#
#     re = /\d\d\d\d-\d\d-\d\d/
#     re.match('1943-02-04')      # => #<MatchData "1943-02-04">
#     re.match('1943-02-04').size # => 1
#     re.match('foo')             # => nil
#
# Adding one or more pairs of parentheses, `(*subexpression*)`, defines
# *groups*, which may result in multiple matched substrings, called *captures*:
#
#     re = /(\d\d\d\d)-(\d\d)-(\d\d)/
#     re.match('1943-02-04')      # => #<MatchData "1943-02-04" 1:"1943" 2:"02" 3:"04">
#     re.match('1943-02-04').size # => 4
#
# The first capture is the entire matched string; the other captures are the
# matched substrings from the groups.
#
# A group may have a [quantifier](rdoc-ref:Regexp@Quantifiers):
#
#     re = /July 4(th)?/
#     re.match('July 4')   # => #<MatchData "July 4" 1:nil>
#     re.match('July 4th') # => #<MatchData "July 4th" 1:"th">
#
#     re = /(foo)*/
#     re.match('')       # => #<MatchData "" 1:nil>
#     re.match('foo')    # => #<MatchData "foo" 1:"foo">
#     re.match('foofoo') # => #<MatchData "foofoo" 1:"foo">
#
#     re = /(foo)+/
#     re.match('')       # => nil
#     re.match('foo')    # => #<MatchData "foo" 1:"foo">
#     re.match('foofoo') # => #<MatchData "foofoo" 1:"foo">
#
# The returned MatchData object gives access to the matched substrings:
#
#     re = /(\d\d\d\d)-(\d\d)-(\d\d)/
#     md = re.match('1943-02-04')
#     # => #<MatchData "1943-02-04" 1:"1943" 2:"02" 3:"04">
#     md[0] # => "1943-02-04"
#     md[1] # => "1943"
#     md[2] # => "02"
#     md[3] # => "04"
#
# #### Non-Capturing Groups
#
# A group may be made non-capturing; it is still a group (and, for example, can
# have a quantifier), but its matching substring is not included among the
# captures.
#
# A non-capturing group begins with `?:` (inside the parentheses):
#
#     # Don't capture the year.
#     re = /(?:\d\d\d\d)-(\d\d)-(\d\d)/
#     md = re.match('1943-02-04') # => #<MatchData "1943-02-04" 1:"02" 2:"04">
#
# #### Backreferences
#
# A group match may also be referenced within the regexp itself; such a
# reference is called a `backreference`:
#
#     /[csh](..) [csh]\1 in/.match('The cat sat in the hat')
#     # => #<MatchData "cat sat in" 1:"at">
#
# This table shows how each subexpression in the regexp above matches a
# substring in the target string:
#
#     | Subexpression in Regexp   | Matching Substring in Target String |
#     |---------------------------|-------------------------------------|
#     |       First '[csh]'       |            Character 'c'            |
#     |          '(..)'           |        First substring 'at'         |
#     |      First space ' '      |      First space character ' '      |
#     |       Second '[csh]'      |            Character 's'            |
#     | '\1' (backreference 'at') |        Second substring 'at'        |
#     |           ' in'           |            Substring ' in'          |
#
# A regexp may contain any number of groups:
#
# *   For a large number of groups:
#
#     *   The ordinary `\*n`* notation applies only for *n* in range (1..9).
#     *   The `MatchData[*n*]` notation applies for any non-negative *n*.
#
# *   `\0` is a special backreference, referring to the entire matched string;
#     it may not be used within the regexp itself, but may be used outside it
#     (for example, in a substitution method call):
#
#         'The cat sat in the hat'.gsub(/[csh]at/, '\0s')
#         # => "The cats sats in the hats"
#
# #### Named Captures
#
# As seen above, a capture can be referred to by its number. A capture can also
# have a name, prefixed as `?<*name*>` or `?'*name*'`, and the name (symbolized)
# may be used as an index in `MatchData[]`:
#
#     md = /\$(?<dollars>\d+)\.(?'cents'\d+)/.match("$3.67")
#     # => #<MatchData "$3.67" dollars:"3" cents:"67">
#     md[:dollars]  # => "3"
#     md[:cents]    # => "67"
#     # The capture numbers are still valid.
#     md[2]         # => "67"
#
# When a regexp contains a named capture, there are no unnamed captures:
#
#     /\$(?<dollars>\d+)\.(\d+)/.match("$3.67")
#     # => #<MatchData "$3.67" dollars:"3">
#
# A named group may be backreferenced as `\k<*name*>`:
#
#     /(?<vowel>[aeiou]).\k<vowel>.\k<vowel>/.match('ototomy')
#     # => #<MatchData "ototo" vowel:"o">
#
# When (and only when) a regexp contains named capture groups and appears before
# the `=~` operator, the captured substrings are assigned to local variables
# with corresponding names:
#
#     /\$(?<dollars>\d+)\.(?<cents>\d+)/ =~ '$3.67'
#     dollars # => "3"
#     cents   # => "67"
#
# Method Regexp#named_captures returns a hash of the capture names and
# substrings; method Regexp#names returns an array of the capture names.
#
# #### Atomic Grouping
#
# A group may be made *atomic* with `(?>`*subexpression*`)`.
#
# This causes the subexpression to be matched independently of the rest of the
# expression, so that the matched substring becomes fixed for the remainder of
# the match, unless the entire subexpression must be abandoned and subsequently
# revisited.
#
# In this way *subexpression* is treated as a non-divisible whole. Atomic
# grouping is typically used to optimise patterns to prevent needless
# backtracking .
#
# Example (without atomic grouping):
#
#     /".*"/.match('"Quote"') # => #<MatchData "\"Quote\"">
#
# Analysis:
#
# 1.  The leading subexpression `"` in the pattern matches the first character
#     `"` in the target string.
# 2.  The next subexpression `.*` matches the next substring `Quote“` (including
#     the trailing double-quote).
# 3.  Now there is nothing left in the target string to match the trailing
#     subexpression `"` in the pattern; this would cause the overall match to
#     fail.
# 4.  The matched substring is backtracked by one position: `Quote`.
# 5.  The final subexpression `"` now matches the final substring `"`, and the
#     overall match succeeds.
#
# If subexpression `.*` is grouped atomically, the backtracking is disabled, and
# the overall match fails:
#
#     /"(?>.*)"/.match('"Quote"') # => nil
#
# Atomic grouping can affect performance; see [Atomic
# Group](https://www.regular-expressions.info/atomic.html).
#
# #### Subexpression Calls
#
# As seen above, a backreference number (`\*n`*) or name (`\k<*name*>`) gives
# access to a captured *substring*; the corresponding regexp *subexpression* may
# also be accessed, via the number (`\\g*n`*) or name (`\g<*name*>`):
#
#     /\A(?<paren>\(\g<paren>*\))*\z/.match('(())')
#     # ^1
#     #      ^2
#     #           ^3
#     #                 ^4
#     #      ^5
#     #           ^6
#     #                      ^7
#     #                       ^8
#     #                       ^9
#     #                           ^10
#
# The pattern:
#
# 1.  Matches at the beginning of the string, i.e. before the first character.
# 2.  Enters a named group `paren`.
# 3.  Matches the first character in the string, `'('`.
# 4.  Calls the `paren` group again, i.e. recurses back to the  second step.
# 5.  Re-enters the `paren` group.
# 6.  Matches the second character in the string, `'('`.
# 7.  Attempts to call `paren` a third time, but fails because doing so would
#     prevent an overall successful match.
# 8.  Matches the third character in the string, `')'`; marks the end of the
#     second recursive call
# 9.  Matches the fourth character in the string, `')'`.
# 10. Matches the end of the string.
#
# See [Subexpression
# calls](https://learnbyexample.github.io/Ruby_Regexp/groupings-and-backreferenc
# es.html?highlight=subexpression#subexpression-calls).
#
# #### Conditionals
#
# The conditional construct takes the form `(?(*cond*)*yes*|*no*)`, where:
#
# *   *cond* may be a capture number or name.
# *   The match to be applied is *yes* if *cond* is captured; otherwise the
#     match to be applied is *no*.
# *   If not needed, `|*no`* may be omitted.
#
# Examples:
#
#     re = /\A(foo)?(?(1)(T)|(F))\z/
#     re.match('fooT') # => #<MatchData "fooT" 1:"foo" 2:"T" 3:nil>
#     re.match('F')    # => #<MatchData "F" 1:nil 2:nil 3:"F">
#     re.match('fooF') # => nil
#     re.match('T')    # => nil
#
#     re = /\A(?<xyzzy>foo)?(?(<xyzzy>)(T)|(F))\z/
#     re.match('fooT') # => #<MatchData "fooT" xyzzy:"foo">
#     re.match('F')    # => #<MatchData "F" xyzzy:nil>
#     re.match('fooF') # => nil
#     re.match('T')    # => nil
#
# #### Absence Operator
#
# The absence operator is a special group that matches anything which does *not*
# match the contained subexpressions.
#
#     /(?~real)/.match('surrealist') # => #<MatchData "surrea">
#     /(?~real)ist/.match('surrealist') # => #<MatchData "ealist">
#     /sur(?~real)ist/.match('surrealist') # => nil
#
# ### Unicode
#
# #### Unicode Properties
#
# The `/\p{*property_name*}/` construct (with lowercase `p`) matches characters
# using a Unicode property name, much like a character class; property `Alpha`
# specifies alphabetic characters:
#
#     /\p{Alpha}/.match('a') # => #<MatchData "a">
#     /\p{Alpha}/.match('1') # => nil
#
# A property can be inverted by prefixing the name with a caret character (`^`):
#
#     /\p{^Alpha}/.match('1') # => #<MatchData "1">
#     /\p{^Alpha}/.match('a') # => nil
#
# Or by using `\P` (uppercase `P`):
#
#     /\P{Alpha}/.match('1') # => #<MatchData "1">
#     /\P{Alpha}/.match('a') # => nil
#
# See [Unicode Properties](rdoc-ref:regexp/unicode_properties.rdoc) for regexps
# based on the numerous properties.
#
# Some commonly-used properties correspond to POSIX bracket expressions:
#
# *   `/\p{Alnum}/`: Alphabetic and numeric character
# *   `/\p{Alpha}/`: Alphabetic character
# *   `/\p{Blank}/`: Space or tab
# *   `/\p{Cntrl}/`: Control character
# *   `/\p{Digit}/`: Digit characters, and similar)
# *   `/\p{Lower}/`: Lowercase alphabetical character
# *   `/\p{Print}/`: Like `\p{Graph}`, but includes the space character
# *   `/\p{Punct}/`: Punctuation character
# *   `/\p{Space}/`: Whitespace character (`[:blank:]`, newline, carriage
#     return, etc.)
# *   `/\p{Upper}/`: Uppercase alphabetical
# *   `/\p{XDigit}/`: Digit allowed in a hexadecimal number (i.e., 0-9a-fA-F)
#
# These are also commonly used:
#
# *   `/\p{Emoji}/`: Unicode emoji.
# *   `/\p{Graph}/`: Non-blank character (excludes spaces, control characters,
#     and similar).
# *   `/\p{Word}/`: A member in one of these Unicode character categories (see
#     below) or having one of these Unicode properties:
#
#     *   Unicode categories:
#         *   `Mark` (`M`).
#         *   `Decimal Number` (`Nd`)
#         *   `Connector Punctuation` (`Pc`).
#
#     *   Unicode properties:
#         *   `Alpha`
#         *   `Join_Control`
#
# *   `/\p{ASCII}/`: A character in the ASCII character set.
# *   `/\p{Any}/`: Any Unicode character (including unassigned characters).
# *   `/\p{Assigned}/`: An assigned character.
#
# #### Unicode Character Categories
#
# A Unicode character category name:
#
# *   May be either its full name or its abbreviated name.
# *   Is case-insensitive.
# *   Treats a space, a hyphen, and an underscore as equivalent.
#
# Examples:
#
#     /\p{lu}/                # => /\p{lu}/
#     /\p{LU}/                # => /\p{LU}/
#     /\p{Uppercase Letter}/  # => /\p{Uppercase Letter}/
#     /\p{Uppercase_Letter}/  # => /\p{Uppercase_Letter}/
#     /\p{UPPERCASE-LETTER}/  # => /\p{UPPERCASE-LETTER}/
#
# Below are the Unicode character category abbreviations and names. Enumerations
# of characters in each category are at the links.
#
# Letters:
#
# *   `L`, `Letter`: `LC`, `Lm`, or `Lo`.
# *   `LC`, `Cased_Letter`: `Ll`, `Lt`, or `Lu`.
# *   [Lu, Lowercase_Letter](https://www.compart.com/en/unicode/category/Ll).
# *   [Lu, Modifier_Letter](https://www.compart.com/en/unicode/category/Lm).
# *   [Lu, Other_Letter](https://www.compart.com/en/unicode/category/Lo).
# *   [Lu, Titlecase_Letter](https://www.compart.com/en/unicode/category/Lt).
# *   [Lu, Uppercase_Letter](https://www.compart.com/en/unicode/category/Lu).
#
# Marks:
#
# *   `M`, `Mark`: `Mc`, `Me`, or `Mn`.
# *   [Mc, Spacing_Mark](https://www.compart.com/en/unicode/category/Mc).
# *   [Me, Enclosing_Mark](https://www.compart.com/en/unicode/category/Me).
# *   [Mn, Nonapacing_Mark](https://www.compart.com/en/unicode/category/Mn).
#
# Numbers:
#
# *   `N`, `Number`: `Nd`, `Nl`, or `No`.
# *   [Nd, Decimal_Number](https://www.compart.com/en/unicode/category/Nd).
# *   [Nl, Letter_Number](https://www.compart.com/en/unicode/category/Nl).
# *   [No, Other_Number](https://www.compart.com/en/unicode/category/No).
#
# Punctuation:
#
# *   `P`, `Punctuation`: `Pc`, `Pd`, `Pe`, `Pf`, `Pi`, `Po`, or `Ps`.
# *   [Pc,
#     Connector_Punctuation](https://www.compart.com/en/unicode/category/Pc).
# *   [Pd, Dash_Punctuation](https://www.compart.com/en/unicode/category/Pd).
# *   [Pe, Close_Punctuation](https://www.compart.com/en/unicode/category/Pe).
# *   [Pf, Final_Punctuation](https://www.compart.com/en/unicode/category/Pf).
# *   [Pi, Initial_Punctuation](https://www.compart.com/en/unicode/category/Pi).
# *   [Po, Other_Punctuation](https://www.compart.com/en/unicode/category/Po).
# *   [Ps, Open_Punctuation](https://www.compart.com/en/unicode/category/Ps).
#
# *   `S`, `Symbol`: `Sc`, `Sk`, `Sm`, or `So`.
# *   [Sc, Currency_Symbol](https://www.compart.com/en/unicode/category/Sc).
# *   [Sk, Modifier_Symbol](https://www.compart.com/en/unicode/category/Sk).
# *   [Sm, Math_Symbol](https://www.compart.com/en/unicode/category/Sm).
# *   [So, Other_Symbol](https://www.compart.com/en/unicode/category/So).
#
# *   `Z`, `Separator`: `Zl`, `Zp`, or `Zs`.
# *   [Zl, Line_Separator](https://www.compart.com/en/unicode/category/Zl).
# *   [Zp, Paragraph_Separator](https://www.compart.com/en/unicode/category/Zp).
# *   [Zs, Space_Separator](https://www.compart.com/en/unicode/category/Zs).
#
# *   `C`, `Other`: `Cc`, `Cf`, `Cn`, `Co`, or `Cs`.
# *   [Cc, Control](https://www.compart.com/en/unicode/category/Cc).
# *   [Cf, Format](https://www.compart.com/en/unicode/category/Cf).
# *   [Cn, Unassigned](https://www.compart.com/en/unicode/category/Cn).
# *   [Co, Private_Use](https://www.compart.com/en/unicode/category/Co).
# *   [Cs, Surrogate](https://www.compart.com/en/unicode/category/Cs).
#
# #### Unicode Scripts and Blocks
#
# Among the Unicode properties are:
#
# *   [Unicode scripts](https://en.wikipedia.org/wiki/Script_(Unicode)); see
#     [supported scripts](https://www.unicode.org/standard/supported.html).
# *   [Unicode blocks](https://en.wikipedia.org/wiki/Unicode_block); see
#     [supported blocks](http://www.unicode.org/Public/UNIDATA/Blocks.txt).
#
# ### POSIX Bracket Expressions
#
# A POSIX *bracket expression* is also similar to a character class. These
# expressions provide a portable alternative to the above, with the added
# benefit of encompassing non-ASCII characters:
#
# *   `/\d/` matches only ASCII decimal digits `0` through `9`.
# *   `/[[:digit:]]/` matches any character in the Unicode `Decimal Number`
#     (`Nd`) category; see below.
#
# The POSIX bracket expressions:
#
# *   `/[[:digit:]]/`: Matches a [Unicode
#     digit](https://www.compart.com/en/unicode/category/Nd):
#
#         /[[:digit:]]/.match('9')       # => #<MatchData "9">
#         /[[:digit:]]/.match("\u1fbf9") # => #<MatchData "9">
#
# *   `/[[:xdigit:]]/`: Matches a digit allowed in a hexadecimal number;
#     equivalent to `[0-9a-fA-F]`.
#
# *   `/[[:upper:]]/`: Matches a [Unicode uppercase
#     letter](https://www.compart.com/en/unicode/category/Lu):
#
#         /[[:upper:]]/.match('A')      # => #<MatchData "A">
#         /[[:upper:]]/.match("\u00c6") # => #<MatchData "Æ">
#
# *   `/[[:lower:]]/`: Matches a [Unicode lowercase
#     letter](https://www.compart.com/en/unicode/category/Ll):
#
#         /[[:lower:]]/.match('a')      # => #<MatchData "a">
#         /[[:lower:]]/.match("\u01fd") # => #<MatchData "ǽ">
#
# *   `/[[:alpha:]]/`: Matches `/[[:upper:]]/` or `/[[:lower:]]/`.
#
# *   `/[[:alnum:]]/`: Matches `/[[:alpha:]]/` or `/[[:digit:]]/`.
#
# *   `/[[:space:]]/`: Matches [Unicode space
#     character](https://www.compart.com/en/unicode/category/Zs):
#
#         /[[:space:]]/.match(' ')      # => #<MatchData " ">
#         /[[:space:]]/.match("\u2005") # => #<MatchData " ">
#
# *   `/[[:blank:]]/`: Matches `/[[:space:]]/` or tab character:
#
#         /[[:blank:]]/.match(' ')      # => #<MatchData " ">
#         /[[:blank:]]/.match("\u2005") # => #<MatchData " ">
#         /[[:blank:]]/.match("\t")     # => #<MatchData "\t">
#
# *   `/[[:cntrl:]]/`: Matches [Unicode control
#     character](https://www.compart.com/en/unicode/category/Cc):
#
#         /[[:cntrl:]]/.match("\u0000") # => #<MatchData "\u0000">
#         /[[:cntrl:]]/.match("\u009f") # => #<MatchData "\u009F">
#
# *   `/[[:graph:]]/`: Matches any character except `/[[:space:]]/` or
#     `/[[:cntrl:]]/`.
#
# *   `/[[:print:]]/`: Matches `/[[:graph:]]/` or space character.
#
# *   `/[[:punct:]]/`: Matches any (Unicode punctuation
#     character}[https://www.compart.com/en/unicode/category/Po]:
#
# Ruby also supports these (non-POSIX) bracket expressions:
#
# *   `/[[:ascii:]]/`: Matches a character in the ASCII character set.
# *   `/[[:word:]]/`: Matches a character in one of these Unicode character
#     categories or having one of these Unicode properties:
#
#     *   Unicode categories:
#         *   `Mark` (`M`).
#         *   `Decimal Number` (`Nd`)
#         *   `Connector Punctuation` (`Pc`).
#
#     *   Unicode properties:
#         *   `Alpha`
#         *   `Join_Control`
#
# ### Comments
#
# A comment may be included in a regexp pattern using the `(?#`*comment*`)`
# construct, where *comment* is a substring that is to be ignored. arbitrary
# text ignored by the regexp engine:
#
#     /foo(?#Ignore me)bar/.match('foobar') # => #<MatchData "foobar">
#
# The comment may not include an unescaped terminator character.
#
# See also [Extended Mode](rdoc-ref:Regexp@Extended+Mode).
#
# ## Modes
#
# Each of these modifiers sets a mode for the regexp:
#
# *   `i`: `/*pattern*/i` sets [Case-Insensitive
#     Mode](rdoc-ref:Regexp@Case-Insensitive+Mode).
# *   `m`: `/*pattern*/m` sets [Multiline Mode](rdoc-ref:Regexp@Multiline+Mode).
# *   `x`: `/*pattern*/x` sets [Extended Mode](rdoc-ref:Regexp@Extended+Mode).
# *   `o`: `/*pattern*/o` sets [Interpolation
#     Mode](rdoc-ref:Regexp@Interpolation+Mode).
#
# Any, all, or none of these may be applied.
#
# Modifiers `i`, `m`, and `x` may be applied to subexpressions:
#
# *   `(?*modifier*)` turns the mode "on" for ensuing subexpressions
# *   `(?-*modifier*)` turns the mode "off" for ensuing subexpressions
# *   `(?*modifier*:*subexp*)` turns the mode "on" for *subexp* within the group
# *   `(?-*modifier*:*subexp*)` turns the mode "off" for *subexp* within the
#     group
#
# Example:
#
#     re = /(?i)te(?-i)st/
#     re.match('test') # => #<MatchData "test">
#     re.match('TEst') # => #<MatchData "TEst">
#     re.match('TEST') # => nil
#     re.match('teST') # => nil
#
#     re = /t(?i:e)st/
#     re.match('test') # => #<MatchData "test">
#     re.match('tEst') # => #<MatchData "tEst">
#     re.match('tEST') # => nil
#
# Method Regexp#options returns an integer whose value showing the settings for
# case-insensitivity mode, multiline mode, and extended mode.
#
# ### Case-Insensitive Mode
#
# By default, a regexp is case-sensitive:
#
#     /foo/.match('FOO')  # => nil
#
# Modifier `i` enables case-insensitive mode:
#
#     /foo/i.match('FOO')
#     # => #<MatchData "FOO">
#
# Method Regexp#casefold? returns whether the mode is case-insensitive.
#
# ### Multiline Mode
#
# The multiline-mode in Ruby is what is commonly called a "dot-all mode":
#
# *   Without the `m` modifier, the subexpression `.` does not match newlines:
#
#         /a.c/.match("a\nc")  # => nil
#
# *   With the modifier, it does match:
#
#         /a.c/m.match("a\nc") # => #<MatchData "a\nc">
#
# Unlike other languages, the modifier `m` does not affect the anchors `^` and
# `$`. These anchors always match at line-boundaries in Ruby.
#
# ### Extended Mode
#
# Modifier `x` enables extended mode, which means that:
#
# *   Literal white space in the pattern is to be ignored.
# *   Character `#` marks the remainder of its containing line as a comment,
#     which is also to be ignored for matching purposes.
#
# In extended mode, whitespace and comments may be used to form a
# self-documented regexp.
#
# Regexp not in extended mode (matches some Roman numerals):
#
#     pattern = '^M{0,3}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})$'
#     re = /#{pattern}/
#     re.match('MCMXLIII') # => #<MatchData "MCMXLIII" 1:"CM" 2:"XL" 3:"III">
#
# Regexp in extended mode:
#
#     pattern = <<-EOT
#       ^                   # beginning of string
#       M{0,3}              # thousands - 0 to 3 Ms
#       (CM|CD|D?C{0,3})    # hundreds - 900 (CM), 400 (CD), 0-300 (0 to 3 Cs),
#                           #            or 500-800 (D, followed by 0 to 3 Cs)
#       (XC|XL|L?X{0,3})    # tens - 90 (XC), 40 (XL), 0-30 (0 to 3 Xs),
#                           #        or 50-80 (L, followed by 0 to 3 Xs)
#       (IX|IV|V?I{0,3})    # ones - 9 (IX), 4 (IV), 0-3 (0 to 3 Is),
#                           #        or 5-8 (V, followed by 0 to 3 Is)
#       $                   # end of string
#     EOT
#     re = /#{pattern}/x
#     re.match('MCMXLIII') # => #<MatchData "MCMXLIII" 1:"CM" 2:"XL" 3:"III">
#
# ### Interpolation Mode
#
# Modifier `o` means that the first time a literal regexp with interpolations is
# encountered, the generated Regexp object is saved and used for all future
# evaluations of that literal regexp. Without modifier `o`, the generated Regexp
# is not saved, so each evaluation of the literal regexp generates a new Regexp
# object.
#
# Without modifier `o`:
#
#     def letters; sleep 5; /[A-Z][a-z]/; end
#     words = %w[abc def xyz]
#     start = Time.now
#     words.each {|word| word.match(/\A[#{letters}]+\z/) }
#     Time.now - start # => 15.0174892
#
# With modifier `o`:
#
#     start = Time.now
#     words.each {|word| word.match(/\A[#{letters}]+\z/o) }
#     Time.now - start # => 5.0010866
#
# Note that if the literal regexp does not have interpolations, the `o` behavior
# is the default.
#
# ## Encodings
#
# By default, a regexp with only US-ASCII characters has US-ASCII encoding:
#
#     re = /foo/
#     re.source.encoding # => #<Encoding:US-ASCII>
#     re.encoding        # => #<Encoding:US-ASCII>
#
# A regular expression containing non-US-ASCII characters is assumed to use the
# source encoding. This can be overridden with one of the following modifiers.
#
# *   `/*pat*/n`: US-ASCII if only containing US-ASCII characters, otherwise
#     ASCII-8BIT:
#
#         /foo/n.encoding     # => #<Encoding:US-ASCII>
#         /foo\xff/n.encoding # => #<Encoding:ASCII-8BIT>
#         /foo\x7f/n.encoding # => #<Encoding:US-ASCII>
#
# *   `/*pat*/u`: UTF-8
#
#         /foo/u.encoding # => #<Encoding:UTF-8>
#
# *   `/*pat*/e`: EUC-JP
#
#         /foo/e.encoding # => #<Encoding:EUC-JP>
#
# *   `/*pat*/s`: Windows-31J
#
#         /foo/s.encoding # => #<Encoding:Windows-31J>
#
# A regexp can be matched against a target string when either:
#
# *   They have the same encoding.
# *   The regexp's encoding is a fixed encoding and the string contains only
#     ASCII characters. Method Regexp#fixed_encoding? returns whether the regexp
#     has a *fixed* encoding.
#
# If a match between incompatible encodings is attempted an
# `Encoding::CompatibilityError` exception is raised.
#
# Example:
#
#     re = eval("# encoding: ISO-8859-1\n/foo\\xff?/")
#     re.encoding                 # => #<Encoding:ISO-8859-1>
#     re =~ "foo".encode("UTF-8") # => 0
#     re =~ "foo\u0100"           # Raises Encoding::CompatibilityError
#
# The encoding may be explicitly fixed by including Regexp::FIXEDENCODING in the
# second argument for Regexp.new:
#
#     # Regexp with encoding ISO-8859-1.
#     re = Regexp.new("a".force_encoding('iso-8859-1'), Regexp::FIXEDENCODING)
#     re.encoding  # => #<Encoding:ISO-8859-1>
#     # Target string with encoding UTF-8.
#     s = "a\u3042"
#     s.encoding   # => #<Encoding:UTF-8>
#     re.match(s)  # Raises Encoding::CompatibilityError.
#
# ## Timeouts
#
# When either a regexp source or a target string comes from untrusted input,
# malicious values could become a denial-of-service attack; to prevent such an
# attack, it is wise to set a timeout.
#
# Regexp has two timeout values:
#
# *   A class default timeout, used for a regexp whose instance timeout is
#     `nil`; this default is initially `nil`, and may be set by method
#     Regexp.timeout=:
#
#         Regexp.timeout # => nil
#         Regexp.timeout = 3.0
#         Regexp.timeout # => 3.0
#
# *   An instance timeout, which defaults to `nil` and may be set in Regexp.new:
#
#         re = Regexp.new('foo', timeout: 5.0)
#         re.timeout # => 5.0
#
# When regexp.timeout is `nil`, the timeout "falls through" to Regexp.timeout;
# when regexp.timeout is non-`nil`, that value controls timing out:
#
#     | regexp.timeout Value | Regexp.timeout Value |            Result           |
#     |----------------------|----------------------|-----------------------------|
#     |         nil          |          nil         |       Never times out.      |
#     |         nil          |         Float        | Times out in Float seconds. |
#     |        Float         |          Any         | Times out in Float seconds. |
#
# ## Optimization
#
# For certain values of the pattern and target string, matching time can grow
# polynomially or exponentially in relation to the input size; the potential
# vulnerability arising from this is the [regular expression
# denial-of-service](https://en.wikipedia.org/wiki/ReDoS) (ReDoS) attack.
#
# Regexp matching can apply an optimization to prevent ReDoS attacks. When the
# optimization is applied, matching time increases linearly (not polynomially or
# exponentially) in relation to the input size, and a ReDoS attach is not
# possible.
#
# This optimization is applied if the pattern meets these criteria:
#
# *   No backreferences.
# *   No subexpression calls.
# *   No nested lookaround anchors or atomic groups.
# *   No nested quantifiers with counting (i.e. no nested `{n}`, `{min,}`,
#     `{,max}`, or `{min,max}` style quantifiers)
#
# You can use method Regexp.linear_time? to determine whether a pattern meets
# these criteria:
#
#     Regexp.linear_time?(/a*/)     # => true
#     Regexp.linear_time?('a*')     # => true
#     Regexp.linear_time?(/(a*)\1/) # => false
#
# However, an untrusted source may not be safe even if the method returns
# `true`, because the optimization uses memoization (which may invoke large
# memory consumption).
#
# ## References
#
# Read (online PDF books):
#
# *   [Mastering Regular
#     Expressions](https://ia902508.us.archive.org/10/items/allitebooks-02/Maste
#     ring%20Regular%20Expressions%2C%203rd%20Edition.pdf) by Jeffrey E.F.
#     Friedl.
# *   [Regular Expressions
#     Cookbook](https://doc.lagout.org/programmation/Regular%20Expressions/Regul
#     ar%20Expressions%20Cookbook_%20Detailed%20Solutions%20in%20Eight%20Program
#     ming%20Languages%20%282nd%20ed.%29%20%5BGoyvaerts%20%26%20Levithan%202012-
#     09-06%5D.pdf) by Jan Goyvaerts & Steven Levithan.
#
# Explore, test (interactive online editor):
#
# *   [Rubular](https://rubular.com/).
#
class Regexp
  # Represents an object's ability to be converted to a `Regexp`.
  #
  # This is only used in `Regexp.try_convert` and `Regexp.union` within the standard library.
  interface _ToRegexp
    # Converts `self` to a `Regexp`.
    def to_regexp: () -> Regexp
  end

  class TimeoutError < RegexpError
  end

  # <!-- rdoc-file=re.c -->
  # see Regexp.options and Regexp.new
  #
  EXTENDED: Integer

  # <!-- rdoc-file=re.c -->
  # see Regexp.options and Regexp.new
  #
  FIXEDENCODING: Integer

  # <!-- rdoc-file=re.c -->
  # see Regexp.options and Regexp.new
  #
  IGNORECASE: Integer

  # <!-- rdoc-file=re.c -->
  # see Regexp.options and Regexp.new
  #
  MULTILINE: Integer

  # <!-- rdoc-file=re.c -->
  # see Regexp.options and Regexp.new
  #
  NOENCODING: Integer

  # <!--
  #   rdoc-file=re.c
  #   - compile(*args)
  # -->
  # Alias for Regexp.new
  #
  alias self.compile self.new

  # <!--
  #   rdoc-file=re.c
  #   - Regexp.escape(string) -> new_string
  # -->
  # Returns a new string that escapes any characters that have special meaning in
  # a regular expression:
  #
  #     s = Regexp.escape('\*?{}.')      # => "\\\\\\*\\?\\{\\}\\."
  #
  # For any string `s`, this call returns a MatchData object:
  #
  #     r = Regexp.new(Regexp.escape(s)) # => /\\\\\\\*\\\?\\\{\\\}\\\./
  #     r.match(s)                       # => #<MatchData "\\\\\\*\\?\\{\\}\\.">
  #
  def self.escape: (interned str) -> String

  # <!--
  #   rdoc-file=re.c
  #   - Regexp.last_match -> matchdata or nil
  #   - Regexp.last_match(n) -> string or nil
  #   - Regexp.last_match(name) -> string or nil
  # -->
  # With no argument, returns the value of `$~`, which is the result of the most
  # recent pattern match (see [Regexp global
  # variables](rdoc-ref:Regexp@Global+Variables)):
  #
  #     /c(.)t/ =~ 'cat'  # => 0
  #     Regexp.last_match # => #<MatchData "cat" 1:"a">
  #     /a/ =~ 'foo'      # => nil
  #     Regexp.last_match # => nil
  #
  # With non-negative integer argument `n`, returns the _n_th field in the
  # matchdata, if any, or nil if none:
  #
  #     /c(.)t/ =~ 'cat'     # => 0
  #     Regexp.last_match(0) # => "cat"
  #     Regexp.last_match(1) # => "a"
  #     Regexp.last_match(2) # => nil
  #
  # With negative integer argument `n`, counts backwards from the last field:
  #
  #     Regexp.last_match(-1)       # => "a"
  #
  # With string or symbol argument `name`, returns the string value for the named
  # capture, if any:
  #
  #     /(?<lhs>\w+)\s*=\s*(?<rhs>\w+)/ =~ 'var = val'
  #     Regexp.last_match        # => #<MatchData "var = val" lhs:"var"rhs:"val">
  #     Regexp.last_match(:lhs)  # => "var"
  #     Regexp.last_match('rhs') # => "val"
  #     Regexp.last_match('foo') # Raises IndexError.
  #
  def self.last_match: () -> MatchData?
                     | (MatchData::capture capture) -> String?

  # <!--
  #   rdoc-file=re.c
  #   - Regexp.linear_time?(re)
  #   - Regexp.linear_time?(string, options = 0)
  # -->
  # Returns `true` if matching against `re` can be done in linear time to the
  # input string.
  #
  #     Regexp.linear_time?(/re/) # => true
  #
  # Note that this is a property of the ruby interpreter, not of the argument
  # regular expression.  Identical regexp can or cannot run in linear time
  # depending on your ruby binary.  Neither forward nor backward compatibility is
  # guaranteed about the return value of this method.  Our current algorithm is
  # (*1) but this is subject to change in the future.  Alternative implementations
  # can also behave differently.  They might always return false for everything.
  #
  # (*1): https://doi.org/10.1109/SP40001.2021.00032
  #
  def self.linear_time?: (Regexp regex, ?nil, ?timeout: untyped) -> bool
                       | (string regex, ?int | string | bool | nil options, ?timeout: untyped) -> bool

  # <!--
  #   rdoc-file=re.c
  #   - Regexp.escape(string) -> new_string
  # -->
  # Returns a new string that escapes any characters that have special meaning in
  # a regular expression:
  #
  #     s = Regexp.escape('\*?{}.')      # => "\\\\\\*\\?\\{\\}\\."
  #
  # For any string `s`, this call returns a MatchData object:
  #
  #     r = Regexp.new(Regexp.escape(s)) # => /\\\\\\\*\\\?\\\{\\\}\\\./
  #     r.match(s)                       # => #<MatchData "\\\\\\*\\?\\{\\}\\.">
  #
  alias self.quote self.escape

  # <!--
  #   rdoc-file=re.c
  #   - Regexp.try_convert(object) -> regexp or nil
  # -->
  # Returns `object` if it is a regexp:
  #
  #     Regexp.try_convert(/re/) # => /re/
  #
  # Otherwise if `object` responds to `:to_regexp`, calls `object.to_regexp` and
  # returns the result.
  #
  # Returns `nil` if `object` does not respond to `:to_regexp`.
  #
  #     Regexp.try_convert('re') # => nil
  #
  # Raises an exception unless `object.to_regexp` returns a regexp.
  #
  def self.try_convert: (Regexp | _ToRegexp regexp_like) -> Regexp
                      | (untyped other) -> Regexp?

  # <!--
  #   rdoc-file=re.c
  #   - Regexp.timeout  -> float or nil
  # -->
  # It returns the current default timeout interval for Regexp matching in second.
  # `nil` means no default timeout configuration.
  #
  def self.timeout: () -> Float?

  # <!--
  #   rdoc-file=re.c
  #   - Regexp.timeout = float or nil
  # -->
  # It sets the default timeout interval for Regexp matching in second. `nil`
  # means no default timeout configuration. This configuration is process-global.
  # If you want to set timeout for each Regexp, use `timeout` keyword for
  # `Regexp.new`.
  #
  #     Regexp.timeout = 1
  #     /^a*b?a*$/ =~ "a" * 100000 + "x" #=> regexp match timeout (RuntimeError)
  #
  def self.timeout=: [T < _ToF] (T timeout) -> T

  # <!--
  #   rdoc-file=re.c
  #   - Regexp.union(*patterns) -> regexp
  #   - Regexp.union(array_of_patterns) -> regexp
  # -->
  # Returns a new regexp that is the union of the given patterns:
  #
  #     r = Regexp.union(%w[cat dog])      # => /cat|dog/
  #     r.match('cat')      # => #<MatchData "cat">
  #     r.match('dog')      # => #<MatchData "dog">
  #     r.match('cog')      # => nil
  #
  # For each pattern that is a string, `Regexp.new(pattern)` is used:
  #
  #     Regexp.union('penzance')             # => /penzance/
  #     Regexp.union('a+b*c')                # => /a\+b\*c/
  #     Regexp.union('skiing', 'sledding')   # => /skiing|sledding/
  #     Regexp.union(['skiing', 'sledding']) # => /skiing|sledding/
  #
  # For each pattern that is a regexp, it is used as is, including its flags:
  #
  #     Regexp.union(/foo/i, /bar/m, /baz/x)
  #     # => /(?i-mx:foo)|(?m-ix:bar)|(?x-mi:baz)/
  #     Regexp.union([/foo/i, /bar/m, /baz/x])
  #     # => /(?i-mx:foo)|(?m-ix:bar)|(?x-mi:baz)/
  #
  # With no arguments, returns `/(?!)/`:
  #
  #     Regexp.union # => /(?!)/
  #
  # If any regexp pattern contains captures, the behavior is unspecified.
  #
  def self.union: (*Regexp | _ToRegexp | string patterns) -> Regexp
                | (array[Regexp | _ToRegexp | string] patterns) -> Regexp
                | (Symbol | [Symbol] symbol_pattern) -> Regexp

  # <!--
  #   rdoc-file=re.c
  #   - Regexp.new(string, options = 0, timeout: nil) -> regexp
  #   - Regexp.new(regexp, timeout: nil) -> regexp
  # -->
  # With argument `string` given, returns a new regexp with the given string and
  # options:
  #
  #     r = Regexp.new('foo') # => /foo/
  #     r.source              # => "foo"
  #     r.options             # => 0
  #
  # Optional argument `options` is one of the following:
  #
  # *   A String of options:
  #
  #         Regexp.new('foo', 'i')  # => /foo/i
  #         Regexp.new('foo', 'im') # => /foo/im
  #
  # *   The bit-wise OR of one or more of the constants Regexp::EXTENDED,
  #     Regexp::IGNORECASE, Regexp::MULTILINE, and Regexp::NOENCODING:
  #
  #         Regexp.new('foo', Regexp::IGNORECASE) # => /foo/i
  #         Regexp.new('foo', Regexp::EXTENDED)   # => /foo/x
  #         Regexp.new('foo', Regexp::MULTILINE)  # => /foo/m
  #         Regexp.new('foo', Regexp::NOENCODING)  # => /foo/n
  #         flags = Regexp::IGNORECASE | Regexp::EXTENDED |  Regexp::MULTILINE
  #         Regexp.new('foo', flags)              # => /foo/mix
  #
  # *   `nil` or `false`, which is ignored.
  # *   Any other truthy value, in which case the regexp will be case-insensitive.
  #
  # If optional keyword argument `timeout` is given, its float value overrides the
  # timeout interval for the class, Regexp.timeout. If `nil` is passed as
  # +timeout, it uses the timeout interval for the class, Regexp.timeout.
  #
  # With argument `regexp` given, returns a new regexp. The source, options,
  # timeout are the same as `regexp`. `options` and `n_flag` arguments are
  # ineffective.  The timeout can be overridden by `timeout` keyword.
  #
  #     options = Regexp::MULTILINE
  #     r = Regexp.new('foo', options, timeout: 1.1) # => /foo/m
  #     r2 = Regexp.new(r)                           # => /foo/m
  #     r2.timeout                                   # => 1.1
  #     r3 = Regexp.new(r, timeout: 3.14)            # => /foo/m
  #     r3.timeout                                   # => 3.14
  #
  def initialize: (Regexp regexp, ?timeout: _ToF?) -> void
                | (string pattern, ?int | string | bool | nil options, ?timeout: _ToF?) -> void

  def initialize_copy: (self object) -> self

  # <!-- rdoc-file=re.c -->
  # Returns `true` if `object` is another Regexp whose pattern, flags, and
  # encoding are the same as `self`, `false` otherwise:
  #
  #     /foo/ == Regexp.new('foo')                          # => true
  #     /foo/ == /foo/i                                     # => false
  #     /foo/ == Regexp.new('food')                         # => false
  #     /foo/ == Regexp.new("abc".force_encoding("euc-jp")) # => false
  #
  def ==: (untyped other) -> bool

  # <!--
  #   rdoc-file=re.c
  #   - regexp === string -> true or false
  # -->
  # Returns `true` if `self` finds a match in `string`:
  #
  #     /^[a-z]*$/ === 'HELLO' # => false
  #     /^[A-Z]*$/ === 'HELLO' # => true
  #
  # This method is called in case statements:
  #
  #     s = 'HELLO'
  #     case s
  #     when /\A[a-z]*\z/; print "Lower case\n"
  #     when /\A[A-Z]*\z/; print "Upper case\n"
  #     else               print "Mixed case\n"
  #     end # => "Upper case"
  #
  def ===: (untyped other) -> bool

  # <!--
  #   rdoc-file=re.c
  #   - regexp =~ string -> integer or nil
  # -->
  # Returns the integer index (in characters) of the first match for `self` and
  # `string`, or `nil` if none; also sets the [rdoc-ref:Regexp global
  # variables](rdoc-ref:Regexp@Global+Variables):
  #
  #     /at/ =~ 'input data' # => 7
  #     $~                   # => #<MatchData "at">
  #     /ax/ =~ 'input data' # => nil
  #     $~                   # => nil
  #
  # Assigns named captures to local variables of the same names if and only if
  # `self`:
  #
  # *   Is a regexp literal; see [Regexp
  #     Literals](rdoc-ref:syntax/literals.rdoc@Regexp+Literals).
  # *   Does not contain interpolations; see [Regexp
  #     interpolation](rdoc-ref:Regexp@Interpolation+Mode).
  # *   Is at the left of the expression.
  #
  # Example:
  #
  #     /(?<lhs>\w+)\s*=\s*(?<rhs>\w+)/ =~ '  x = y  '
  #     p lhs # => "x"
  #     p rhs # => "y"
  #
  # Assigns `nil` if not matched:
  #
  #     /(?<lhs>\w+)\s*=\s*(?<rhs>\w+)/ =~ '  x = '
  #     p lhs # => nil
  #     p rhs # => nil
  #
  # Does not make local variable assignments if `self` is not a regexp literal:
  #
  #     r = /(?<foo>\w+)\s*=\s*(?<foo>\w+)/
  #     r =~ '  x = y  '
  #     p foo # Undefined local variable
  #     p bar # Undefined local variable
  #
  # The assignment does not occur if the regexp is not at the left:
  #
  #     '  x = y  ' =~ /(?<foo>\w+)\s*=\s*(?<foo>\w+)/
  #     p foo, foo # Undefined local variables
  #
  # A regexp interpolation, `#{}`, also disables the assignment:
  #
  #     r = /(?<foo>\w+)/
  #     /(?<foo>\w+)\s*=\s*#{r}/ =~ 'x = y'
  #     p foo # Undefined local variable
  #
  def =~: (interned? string) -> Integer?
        | (nil) -> nil

  # <!--
  #   rdoc-file=re.c
  #   - casefold?-> true or false
  # -->
  # Returns `true` if the case-insensitivity flag in `self` is set, `false`
  # otherwise:
  #
  #     /a/.casefold?           # => false
  #     /a/i.casefold?          # => true
  #     /(?i:a)/.casefold?      # => false
  #
  def casefold?: () -> bool

  # <!--
  #   rdoc-file=re.c
  #   - obj.encoding   -> encoding
  # -->
  # Returns the Encoding object that represents the encoding of obj.
  #
  def encoding: () -> Encoding

  # <!--
  #   rdoc-file=re.c
  #   - regexp == object -> true or false
  # -->
  # Returns `true` if `object` is another Regexp whose pattern, flags, and
  # encoding are the same as `self`, `false` otherwise:
  #
  #     /foo/ == Regexp.new('foo')                          # => true
  #     /foo/ == /foo/i                                     # => false
  #     /foo/ == Regexp.new('food')                         # => false
  #     /foo/ == Regexp.new("abc".force_encoding("euc-jp")) # => false
  #
  alias eql? ==

  # <!--
  #   rdoc-file=re.c
  #   - fixed_encoding?   -> true or false
  # -->
  # Returns `false` if `self` is applicable to a string with any ASCII-compatible
  # encoding; otherwise returns `true`:
  #
  #     r = /a/                                          # => /a/
  #     r.fixed_encoding?                               # => false
  #     r.match?("\u{6666} a")                          # => true
  #     r.match?("\xa1\xa2 a".force_encoding("euc-jp")) # => true
  #     r.match?("abc".force_encoding("euc-jp"))        # => true
  #
  #     r = /a/u                                        # => /a/
  #     r.fixed_encoding?                               # => true
  #     r.match?("\u{6666} a")                          # => true
  #     r.match?("\xa1\xa2".force_encoding("euc-jp"))   # Raises exception.
  #     r.match?("abc".force_encoding("euc-jp"))        # => true
  #
  #     r = /\u{6666}/                                  # => /\u{6666}/
  #     r.fixed_encoding?                               # => true
  #     r.encoding                                      # => #<Encoding:UTF-8>
  #     r.match?("\u{6666} a")                          # => true
  #     r.match?("\xa1\xa2".force_encoding("euc-jp"))   # Raises exception.
  #     r.match?("abc".force_encoding("euc-jp"))        # => false
  #
  def fixed_encoding?: () -> bool

  # <!--
  #   rdoc-file=re.c
  #   - hash -> integer
  # -->
  # Returns the integer hash value for `self`.
  #
  # Related: Object#hash.
  #
  def hash: () -> Integer

  # <!--
  #   rdoc-file=re.c
  #   - inspect -> string
  # -->
  # Returns a nicely-formatted string representation of `self`:
  #
  #     /ab+c/ix.inspect # => "/ab+c/ix"
  #
  # Related: Regexp#to_s.
  #
  def inspect: () -> String

  # <!--
  #   rdoc-file=re.c
  #   - match(string, offset = 0) -> matchdata or nil
  #   - match(string, offset = 0) {|matchdata| ... } -> object
  # -->
  # With no block given, returns the MatchData object that describes the match, if
  # any, or `nil` if none; the search begins at the given character `offset` in
  # `string`:
  #
  #     /abra/.match('abracadabra')      # => #<MatchData "abra">
  #     /abra/.match('abracadabra', 4)   # => #<MatchData "abra">
  #     /abra/.match('abracadabra', 8)   # => nil
  #     /abra/.match('abracadabra', 800) # => nil
  #
  #     string = "\u{5d0 5d1 5e8 5d0}cadabra"
  #     /abra/.match(string, 7)          #=> #<MatchData "abra">
  #     /abra/.match(string, 8)          #=> nil
  #     /abra/.match(string.b, 8)        #=> #<MatchData "abra">
  #
  # With a block given, calls the block if and only if a match is found; returns
  # the block's value:
  #
  #     /abra/.match('abracadabra') {|matchdata| p matchdata }
  #     # => #<MatchData "abra">
  #     /abra/.match('abracadabra', 4) {|matchdata| p matchdata }
  #     # => #<MatchData "abra">
  #     /abra/.match('abracadabra', 8) {|matchdata| p matchdata }
  #     # => nil
  #     /abra/.match('abracadabra', 8) {|marchdata| fail 'Cannot happen' }
  #     # => nil
  #
  # Output (from the first two blocks above):
  #
  #     #<MatchData "abra">
  #     #<MatchData "abra">
  #
  #      /(.)(.)(.)/.match("abc")[2] # => "b"
  #      /(.)(.)/.match("abc", 1)[2] # => "c"
  #
  def match: (interned? str, ?int offset) -> MatchData?
           | [T] (interned? str, ?int offset) { (MatchData matchdata) -> T } -> T?
           | (nil, ?int offset) ?{ (MatchData matchdata) -> void } -> nil

  # <!--
  #   rdoc-file=re.c
  #   - match?(string) -> true or false
  #   - match?(string, offset = 0) -> true or false
  # -->
  # Returns `true` or `false` to indicate whether the regexp is matched or not
  # without updating $~ and other related variables. If the second parameter is
  # present, it specifies the position in the string to begin the search.
  #
  #     /R.../.match?("Ruby")    # => true
  #     /R.../.match?("Ruby", 1) # => false
  #     /P.../.match?("Ruby")    # => false
  #     $&                       # => nil
  #
  def match?: (interned str, ?int offset) -> bool
            | (nil, ?int offset) -> false

  # <!--
  #   rdoc-file=re.c
  #   - named_captures  -> hash
  # -->
  # Returns a hash representing named captures of `self` (see [Named
  # Captures](rdoc-ref:Regexp@Named+Captures)):
  #
  # *   Each key is the name of a named capture.
  # *   Each value is an array of integer indexes for that named capture.
  #
  # Examples:
  #
  #     /(?<foo>.)(?<bar>.)/.named_captures # => {"foo"=>[1], "bar"=>[2]}
  #     /(?<foo>.)(?<foo>.)/.named_captures # => {"foo"=>[1, 2]}
  #     /(.)(.)/.named_captures             # => {}
  #
  def named_captures: () -> Hash[String, Array[Integer]]

  # <!--
  #   rdoc-file=re.c
  #   - names -> array_of_names
  # -->
  # Returns an array of names of captures (see [Named
  # Captures](rdoc-ref:Regexp@Named+Captures)):
  #
  #     /(?<foo>.)(?<bar>.)(?<baz>.)/.names # => ["foo", "bar", "baz"]
  #     /(?<foo>.)(?<foo>.)/.names          # => ["foo"]
  #     /(.)(.)/.names                      # => []
  #
  def names: () -> Array[String]

  # <!--
  #   rdoc-file=re.c
  #   - options -> integer
  # -->
  # Returns an integer whose bits show the options set in `self`.
  #
  # The option bits are:
  #
  #     Regexp::IGNORECASE # => 1
  #     Regexp::EXTENDED   # => 2
  #     Regexp::MULTILINE  # => 4
  #
  # Examples:
  #
  #     /foo/.options    # => 0
  #     /foo/i.options   # => 1
  #     /foo/x.options   # => 2
  #     /foo/m.options   # => 4
  #     /foo/mix.options # => 7
  #
  # Note that additional bits may be set in the returned integer; these are
  # maintained internally in `self`, are ignored if passed to Regexp.new, and may
  # be ignored by the caller:
  #
  # Returns the set of bits corresponding to the options used when creating this
  # regexp (see Regexp::new for details). Note that additional bits may be set in
  # the returned options: these are used internally by the regular expression
  # code. These extra bits are ignored if the options are passed to Regexp::new:
  #
  #     r = /\xa1\xa2/e                 # => /\xa1\xa2/
  #     r.source                        # => "\\xa1\\xa2"
  #     r.options                       # => 16
  #     Regexp.new(r.source, r.options) # => /\xa1\xa2/
  #
  def options: () -> Integer

  # <!--
  #   rdoc-file=re.c
  #   - source -> string
  # -->
  # Returns the original string of `self`:
  #
  #     /ab+c/ix.source # => "ab+c"
  #
  # Regexp escape sequences are retained:
  #
  #     /\x20\+/.source  # => "\\x20\\+"
  #
  # Lexer escape characters are not retained:
  #
  #     /\//.source  # => "/"
  #
  def source: () -> String

  # <!--
  #   rdoc-file=re.c
  #   - to_s -> string
  # -->
  # Returns a string showing the options and string of `self`:
  #
  #     r0 = /ab+c/ix
  #     s0 = r0.to_s # => "(?ix-m:ab+c)"
  #
  # The returned string may be used as an argument to Regexp.new, or as
  # interpolated text for a [Regexp
  # interpolation](rdoc-ref:Regexp@Interpolation+Mode):
  #
  #     r1 = Regexp.new(s0) # => /(?ix-m:ab+c)/
  #     r2 = /#{s0}/        # => /(?ix-m:ab+c)/
  #
  # Note that `r1` and `r2` are not equal to `r0` because their original strings
  # are different:
  #
  #     r0 == r1  # => false
  #     r0.source # => "ab+c"
  #     r1.source # => "(?ix-m:ab+c)"
  #
  # Related: Regexp#inspect.
  #
  def to_s: () -> String

  # <!--
  #   rdoc-file=re.c
  #   - rxp.timeout  -> float or nil
  # -->
  # It returns the timeout interval for Regexp matching in second. `nil` means no
  # default timeout configuration.
  #
  # This configuration is per-object. The global configuration set by
  # Regexp.timeout= is ignored if per-object configuration is set.
  #
  #     re = Regexp.new("^a*b?a*$", timeout: 1)
  #     re.timeout               #=> 1.0
  #     re =~ "a" * 100000 + "x" #=> regexp match timeout (RuntimeError)
  #
  %a{pure}
  def timeout: () -> Float?

  # <!--
  #   rdoc-file=re.c
  #   - ~ rxp -> integer or nil
  # -->
  # Equivalent to *`rxp* =~ $_`:
  #
  #     $_ = "input data"
  #     ~ /at/ # => 7
  #
  def ~: () -> Integer?
end
