[DOC] Fix doc/regexp.rdoc links

- Rename regexp.rdoc to exclude from "Pages".  This file is for to be
  included in the "class Regexp" document, but it also appeared as a
  separate page duplicately.
- Fix links on case-sensitive filesystems.
- Fix to use rdoc-ref instead of converted HTML page names.

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+A {regular expression}[https://en.wikipedia.org/wiki/Regular_expression]
+(also called a _regexp_) is a <i>match pattern</i> (also simply called a _pattern_).
+
+A common notation for a regexp uses enclosing slash characters:
+
+  /foo/
+
+A regexp may be applied to a <i>target string</i>;
+The part of the string (if any) that matches the pattern is called a _match_,
+and may be said <i>to match</i>:
+
+  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 <tt>%r</tt> 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 <tt>match</tt>
+
+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 <tt>=~</tt>
+
+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 <tt>match?</tt>
+
+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:
+
+- <tt>#match</tt>: see {Method match}[rdoc-ref:Regexp@Method+match].
+- <tt>#=~</tt>: see {Operator =~}[rdoc-ref:Regexp@Operator+-3D~].
+
+The affected global variables are:
+
+- <tt>$~</tt>: Returns a MatchData object, or +nil+.
+- <tt>$&</tt>: Returns the matched part of the string, or +nil+.
+- <tt>$`</tt>: Returns the part of the string to the left of the match, or +nil+.
+- <tt>$'</tt>: Returns the part of the string to the right of the match, or +nil+.
+- <tt>$+</tt>: Returns the last group matched, or +nil+.
+- <tt>$1</tt>, <tt>$2</tt>, etc.: Returns the first, second, etc.,
+  matched group, or +nil+.
+  Note that <tt>$0</tt> 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 {String Literals}[rdoc-ref:syntax/literals.rdoc@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].
+
+- <tt>\s</tt> 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 <i>character class</i> 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 (<tt>^</tt>),
+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 <tt>[a-z[0-9]]</tt>
+describes the same set as <tt>[a-z0-9]</tt>.
+
+However, character classes also support the <tt>&&</tt> 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:
+
+- <tt>/./</tt>: Matches any character except a newline:
+
+    /./.match('foo') # => #<MatchData "f">
+    /./.match("\n")  # => nil
+
+- <tt>/./m</tt>: Matches any character, including a newline;
+  see {Multiline Mode}[rdoc-ref:Regexp@Multiline+Mode]:
+
+    /./m.match("\n") # => #<MatchData "\n">
+
+- <tt>/\w/</tt>: Matches a word character: equivalent to <tt>[a-zA-Z0-9_]</tt>:
+
+    /\w/.match(' foo') # => #<MatchData "f">
+    /\w/.match(' _')   # => #<MatchData "_">
+    /\w/.match(' ')    # => nil
+
+- <tt>/\W/</tt>: Matches a non-word character: equivalent to <tt>[^a-zA-Z0-9_]</tt>:
+
+    /\W/.match(' ') # => #<MatchData " ">
+    /\W/.match('_') # => nil
+
+- <tt>/\d/</tt>: Matches a digit character: equivalent to <tt>[0-9]</tt>:
+
+    /\d/.match('THX1138') # => #<MatchData "1">
+    /\d/.match('foo')     # => nil
+
+- <tt>/\D/</tt>: Matches a non-digit character: equivalent to <tt>[^0-9]</tt>:
+
+    /\D/.match('123Jump!') # => #<MatchData "J">
+    /\D/.match('123')      # => nil
+
+- <tt>/\h/</tt>: Matches a hexdigit character: equivalent to <tt>[0-9a-fA-F]</tt>:
+
+    /\h/.match('xyz fedcba9876543210') # => #<MatchData "f">
+    /\h/.match('xyz')                  # => nil
+
+- <tt>/\H/</tt>: Matches a non-hexdigit character: equivalent to <tt>[^0-9a-fA-F]</tt>:
+
+    /\H/.match('fedcba9876543210xyz') # => #<MatchData "x">
+    /\H/.match('fedcba9876543210')    # => nil
+
+- <tt>/\s/</tt>: Matches a whitespace character: equivalent to <tt>/[ \t\r\n\f\v]/</tt>:
+
+    /\s/.match('foo bar') # => #<MatchData " ">
+    /\s/.match('foo')     # => nil
+
+- <tt>/\S/</tt>: Matches a non-whitespace character: equivalent to <tt>/[^ \t\r\n\f\v]/</tt>:
+
+    /\S/.match(" \t\r\n\f\v foo") # => #<MatchData "f">
+    /\S/.match(" \t\r\n\f\v")     # => nil
+
+- <tt>/\R/</tt>: 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:
+
+- <tt>^</tt>: Matches the beginning of a line:
+
+    /^bar/.match("foo\nbar") # => #<MatchData "bar">
+    /^ar/.match("foo\nbar")  # => nil
+
+- <tt>$</tt>: Matches the end of a line:
+
+    /bar$/.match("foo\nbar") # => #<MatchData "bar">
+    /ba$/.match("foo\nbar")  # => nil
+
+- <tt>\A</tt>: Matches the beginning of the string:
+
+    /\Afoo/.match('foo bar')  # => #<MatchData "foo">
+    /\Afoo/.match(' foo bar') # => nil
+
+- <tt>\Z</tt>: 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
+
+- <tt>\z</tt>: 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
+
+- <tt>\b</tt>: Matches word boundary when not inside brackets;
+  matches backspace (<tt>"0x08"</tt>) when inside brackets:
+
+    /foo\b/.match('foo bar') # => #<MatchData "foo">
+    /foo\b/.match('foobar')  # => nil
+
+- <tt>\B</tt>: Matches non-word boundary:
+
+    /foo\B/.match('foobar')  # => #<MatchData "foo">
+    /foo\B/.match('foo bar') # => nil
+
+- <tt>\G</tt>: 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:
+
+- <tt>(?=_pat_)</tt>: Positive lookahead assertion:
+  ensures that the following characters match _pat_,
+  but doesn't include those characters in the matched substring.
+
+- <tt>(?!_pat_)</tt>: Negative lookahead assertion:
+  ensures that the following characters <i>do not</i> match _pat_,
+  but doesn't include those characters in the matched substring.
+
+Lookbehind anchors:
+
+- <tt>(?<=_pat_)</tt>: Positive lookbehind assertion:
+  ensures that the preceding characters match _pat_, but
+  doesn't include those characters in the matched substring.
+
+- <tt>(?<!_pat_)</tt>: 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 <tt><b></tt>...<tt></b></tt> tags
+without including the tags in the match:
+
+  /(?<=<b>)\w+(?=<\/b>)/.match("Fortune favors the <b>bold</b>.")
+  # => #<MatchData "bold">
+
+==== Match-Reset Anchor
+
+- <tt>\K</tt>: Match reset:
+  the matched content preceding <tt>\K</tt> 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 <tt>$&</tt> equals <tt>'c'</tt>,
+  while the matched position is different.
+
+  As are the following two regexps:
+
+    /(a)\K(b)\Kc/
+    /(?<=(?<=(a))(b))c/
+
+=== Alternation
+
+The vertical bar metacharacter (<tt>|</tt>) 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:
+
+- <tt>*</tt> - Matches zero or more times:
+
+    /\w*/.match('')
+    # => #<MatchData "">
+    /\w*/.match('x')
+    # => #<MatchData "x">
+    /\w*/.match('xyz')
+    # => #<MatchData "yz">
+
+- <tt>+</tt> - Matches one or more times:
+
+    /\w+/.match('')    # => nil
+    /\w+/.match('x')   # => #<MatchData "x">
+    /\w+/.match('xyz') # => #<MatchData "xyz">
+
+- <tt>?</tt> - Matches zero or one times:
+
+    /\w?/.match('')    # => #<MatchData "">
+    /\w?/.match('x')   # => #<MatchData "x">
+    /\w?/.match('xyz') # => #<MatchData "x">
+
+- <tt>{</tt>_n_<tt>}</tt> - Matches exactly _n_ times:
+
+    /\w{2}/.match('')    # => nil
+    /\w{2}/.match('x')   # => nil
+    /\w{2}/.match('xyz') # => #<MatchData "xy">
+
+- <tt>{</tt>_min_<tt>,}</tt> - 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">
+
+- <tt>{,</tt>_max_<tt>}</tt> - Matches _max_ or fewer times:
+
+    /\w{,2}/.match('')    # => #<MatchData "">
+    /\w{,2}/.match('x')   # => #<MatchData "x">
+    /\w{,2}/.match('xyz') # => #<MatchData "xy">
+
+- <tt>{</tt>_min_<tt>,</tt>_max_<tt>}</tt> -
+  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: <tt>*</tt>, <tt>+</tt>, <tt>?</tt>,
+  <tt>{min, max}</tt> and its variants.
+- In _lazy_ matching, the minimum number of occurrences are matched.
+  Lazy quantifiers: <tt>*?</tt>, <tt>+?</tt>, <tt>??</tt>,
+  <tt>{min, max}?</tt> 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: <tt>*+</tt>, <tt>++</tt>, <tt>?+</tt>.
+  Note that <tt>{min, max}</tt> 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/Regular%20Expressions%20Cookbook_%20Detailed%20Solutions%20in%20Eight%20Programming%20Languages%20%282nd%20ed.%29%20%5BGoyvaerts%20%26%20Levithan%202012-09-06%5D.pdf#tutorial-backtrack].
+- About possessive matching, see
+  {Eliminate Needless Backtracking}[https://doc.lagout.org/programmation/Regular%20Expressions/Regular%20Expressions%20Cookbook_%20Detailed%20Solutions%20in%20Eight%20Programming%20Languages%20%282nd%20ed.%29%20%5BGoyvaerts%20%26%20Levithan%202012-09-06%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, <tt>(_subexpression_)</tt>,
+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 <tt>?:</tt> (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 <tt>\\_n_</tt> notation applies only for _n_ in range (1..9).
+  - The <tt>MatchData[_n_]</tt> notation applies for any non-negative _n_.
+
+- <tt>\0</tt> 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 <tt>?<_name_></tt> or <tt>?'_name_'</tt>,
+and the name (symbolized) may be used as an index in <tt>MatchData[]</tt>:
+
+  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 <tt>\k<_name_></tt>:
+
+  /(?<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 <tt>=~</tt> 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 <tt>(?></tt>_subexpression_<tt>)</tt>.
+
+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 <tt>"</tt> in the pattern matches the first character
+   <tt>"</tt> in the target string.
+2. The next subexpression <tt>.*</tt> matches the next substring <tt>Quote“</tt>
+   (including the trailing double-quote).
+3. Now there is nothing left in the target string to match
+   the trailing subexpression <tt>"</tt> in the pattern;
+   this would cause the overall match to fail.
+4. The matched substring is backtracked by one position: <tt>Quote</tt>.
+5. The final subexpression <tt>"</tt> now matches the final substring <tt>"</tt>,
+   and the overall match succeeds.
+
+If subexpression <tt>.*</tt> 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 (<tt>\\_n_</tt>) or name (<tt>\k<_name_></tt>)
+gives access to a captured _substring_;
+the corresponding regexp _subexpression_ may also be accessed,
+via the number (<tt>\\g<i>n</i></tt>) or name (<tt>\g<_name_></tt>):
+
+  /\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, <tt>'('</tt>.
+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, <tt>'('</tt>.
+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, <tt>')'</tt>;
+    marks the end of the second recursive call
+9.  Matches the fourth character in the string, <tt>')'</tt>.
+10. Matches the end of the string.
+
+See {Subexpression calls}[https://learnbyexample.github.io/Ruby_Regexp/groupings-and-backreferences.html?highlight=subexpression#subexpression-calls].
+
+==== Conditionals
+
+The conditional construct takes the form <tt>(?(_cond_)_yes_|_no_)</tt>, 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, <tt>|_no_</tt> 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 <tt>/\p{_property_name_}/</tt> 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 (<tt>^</tt>):
+
+  /\p{^Alpha}/.match('1') # => #<MatchData "1">
+  /\p{^Alpha}/.match('a') # => nil
+
+Or by using <tt>\P</tt> (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:
+
+- <tt>/\p{Alnum}/</tt>: Alphabetic and numeric character
+- <tt>/\p{Alpha}/</tt>: Alphabetic character
+- <tt>/\p{Blank}/</tt>: Space or tab
+- <tt>/\p{Cntrl}/</tt>: Control character
+- <tt>/\p{Digit}/</tt>: Digit
+  characters, and similar)
+- <tt>/\p{Lower}/</tt>: Lowercase alphabetical character
+- <tt>/\p{Print}/</tt>: Like <tt>\p{Graph}</tt>, but includes the space character
+- <tt>/\p{Punct}/</tt>: Punctuation character
+- <tt>/\p{Space}/</tt>: Whitespace character (<tt>[:blank:]</tt>, newline,
+  carriage return, etc.)
+- <tt>/\p{Upper}/</tt>: Uppercase alphabetical
+- <tt>/\p{XDigit}/</tt>: Digit allowed in a hexadecimal number (i.e., 0-9a-fA-F)
+
+These are also commonly used:
+
+- <tt>/\p{Emoji}/</tt>: Unicode emoji.
+- <tt>/\p{Graph}/</tt>: Non-blank character
+  (excludes spaces, control characters, and similar).
+- <tt>/\p{Word}/</tt>: A member of one of the following Unicode character
+  categories (see below):
+
+  - +Mark+ (+M+).
+  - +Letter+ (+L+).
+  - +Number+ (+N+)
+  - <tt>Connector Punctuation</tt> (+Pc+).
+
+- <tt>/\p{ASCII}/</tt>: A character in the ASCII character set.
+- <tt>/\p{Any}/</tt>: Any Unicode character (including unassigned characters).
+- <tt>/\p{Assigned}/</tt>: 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].
+
+Punctation:
+
+- +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 <i>bracket expression</i> 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:
+
+- <tt>/\d/</tt> matches only ASCII decimal digits +0+ through +9+.
+- <tt>/[[:digit:]]/</tt> matches any character in the Unicode
+  <tt>Decimal Number</tt> (+Nd+) category;
+  see below.
+
+The POSIX bracket expressions:
+
+- <tt>/[[:digit:]]/</tt>: Matches a {Unicode digit}[https://www.compart.com/en/unicode/category/Nd]:
+
+    /[[:digit:]]/.match('9')       # => #<MatchData "9">
+    /[[:digit:]]/.match("\u1fbf9") # => #<MatchData "9">
+
+- <tt>/[[:xdigit:]]/</tt>: Matches a digit allowed in a hexadecimal number;
+  equivalent to <tt>[0-9a-fA-F]</tt>.
+
+- <tt>/[[:upper:]]/</tt>: Matches a {Unicode uppercase letter}[https://www.compart.com/en/unicode/category/Lu]:
+
+    /[[:upper:]]/.match('A')      # => #<MatchData "A">
+    /[[:upper:]]/.match("\u00c6") # => #<MatchData "Æ">
+
+- <tt>/[[:lower:]]/</tt>: Matches a {Unicode lowercase letter}[https://www.compart.com/en/unicode/category/Ll]:
+
+    /[[:lower:]]/.match('a')      # => #<MatchData "a">
+    /[[:lower:]]/.match("\u01fd") # => #<MatchData "ǽ">
+
+- <tt>/[[:alpha:]]/</tt>: Matches <tt>/[[:upper:]]/</tt> or <tt>/[[:lower:]]/</tt>.
+
+- <tt>/[[:alnum:]]/</tt>: Matches <tt>/[[:alpha:]]/</tt> or <tt>/[[:digit:]]/</tt>.
+
+- <tt>/[[:space:]]/</tt>: Matches {Unicode space character}[https://www.compart.com/en/unicode/category/Zs]:
+
+    /[[:space:]]/.match(' ')      # => #<MatchData " ">
+    /[[:space:]]/.match("\u2005") # => #<MatchData " ">
+
+- <tt>/[[:blank:]]/</tt>: Matches <tt>/[[:space:]]/</tt> or tab character:
+
+    /[[:blank:]]/.match(' ')      # => #<MatchData " ">
+    /[[:blank:]]/.match("\u2005") # => #<MatchData " ">
+    /[[:blank:]]/.match("\t")     # => #<MatchData "\t">
+
+- <tt>/[[:cntrl:]]/</tt>: Matches {Unicode control character}[https://www.compart.com/en/unicode/category/Cc]:
+
+    /[[:cntrl:]]/.match("\u0000") # => #<MatchData "\u0000">
+    /[[:cntrl:]]/.match("\u009f") # => #<MatchData "\u009F">
+
+- <tt>/[[:graph:]]/</tt>: Matches any character
+  except <tt>/[[:space:]]/</tt> or <tt>/[[:cntrl:]]/</tt>.
+
+- <tt>/[[:print:]]/</tt>: Matches <tt>/[[:graph:]]/</tt> or space character.
+
+- <tt>/[[:punct:]]/</tt>: Matches any (Unicode punctuation character}[https://www.compart.com/en/unicode/category/Po]:
+
+Ruby also supports these (non-POSIX) bracket expressions:
+
+- <tt>/[[:ascii:]]/</tt>: Matches a character in the ASCII character set.
+- <tt>/[[:word:]]/</tt>: Matches a character in one of these Unicode character
+  categories (see below):
+
+  - +Mark+ (+M+).
+  - +Letter+ (+L+).
+  - +Number+ (+N+)
+  - <tt>Connector Punctuation</tt> (+Pc+).
+
+=== Comments
+
+A comment may be included in a regexp pattern
+using the <tt>(?#</tt>_comment_<tt>)</tt> 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+: <tt>/_pattern_/i</tt> sets
+  {Case-Insensitive Mode}[rdoc-ref:Regexp@Case-Insensitive+Mode].
+- +m+: <tt>/_pattern_/m</tt> sets
+  {Multiline Mode}[rdoc-ref:Regexp@Multiline+Mode].
+- +x+: <tt>/_pattern_/x</tt> sets
+  {Extended Mode}[rdoc-ref:Regexp@Extended+Mode].
+- +o+: <tt>/_pattern_/o</tt> 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:
+
+- <tt>(?_modifier_)</tt> turns the mode "on" for ensuing subexpressions
+- <tt>(?-_modifier_)</tt> turns the mode "off" for ensuing subexpressions
+- <tt>(?_modifier_:_subexp_)</tt> turns the mode "on" for _subexp_ within the group
+- <tt>(?-_modifier_:_subexp_)</tt> 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 <tt>.</tt> 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 <tt>^</tt> and <tt>$</tt>.
+These anchors always match at line-boundaries in Ruby.
+
+=== Extended Mode
+
+Modifier +x+ enables extended mode, which means that:
+
+- Literal white space in the pattern is to be ignored.
+- Character <tt>#</tt> marks the remainder of its containing line as a comment,
+  which is also to be ignored for matching purposes.
+
+In extended mode, whitespace and comments may be used
+to form a self-documented regexp.
+
+Regexp not in extended mode (matches some Roman numerals):
+
+  pattern = '^M{0,3}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})$'
+  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.
+
+- <tt>/_pat_/n</tt>: 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>
+
+- <tt>/_pat_/u</tt>: UTF-8
+
+    /foo/u.encoding # => #<Encoding:UTF-8>
+
+- <tt>/_pat_/e</tt>: EUC-JP
+
+    /foo/e.encoding # => #<Encoding:EUC-JP>
+
+- <tt>/_pat_/s</tt>: 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 <i>fixed</i> encoding.
+
+If a match between incompatible encodings is attempted an
+<tt>Encoding::CompatibilityError</tt> 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 <tt>{n}</tt>,
+  <tt>{min,}</tt>, <tt>{,max}</tt>, or <tt>{min,max}</tt> 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/Mastering%20Regular%20Expressions%2C%203rd%20Edition.pdf]
+  by Jeffrey E.F. Friedl.
+- {Regular Expressions Cookbook}[https://doc.lagout.org/programmation/Regular%20Expressions/Regular%20Expressions%20Cookbook_%20Detailed%20Solutions%20in%20Eight%20Programming%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/].