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:language/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.

These variables, except for <tt>$~</tt>, are shorthands for methods of
<tt>$~</tt>.  See MatchData@Global+variables+equivalence.

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].

- <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">

The pattern in lookbehind must be fixed-width.
But top-level alternatives can be of various lengths.
ex. (?<=a|bc) is OK. (?<=aaa(?:b|cd)) is not allowed.

==== 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 "xyz">

- <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:language/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>: Characters excluding <tt>/\p{Cntrl}/</tt> and <tt>/\p{Space}/</tt>.
  Note that invisible characters under the Unicode
  {"Format"}[https://www.compart.com/en/unicode/category/Cf] category are included.
- <tt>/\p{Word}/</tt>: A member in one of these Unicode character
  categories (see below) or having one of these Unicode properties:

  - Unicode categories:
    - +Mark+ (+M+).
    - <tt>Decimal Number</tt> (+Nd+)
    - <tt>Connector Punctuation</tt> (+Pc+).

  - Unicode properties:
    - +Alpha+
    - <tt>Join_Control</tt>

- <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].

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 <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 or having one of these Unicode properties:

  - Unicode categories:
    - +Mark+ (+M+).
    - <tt>Decimal Number</tt> (+Nd+)
    - <tt>Connector Punctuation</tt> (+Pc+).

  - Unicode properties:
    - +Alpha+
    - <tt>Join_Control</tt>

=== 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 attack 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:

- <i>Mastering Regular Expressions</i>
  by Jeffrey E.F. Friedl.
- <i>Regular Expressions Cookbook</i>
  by Jan Goyvaerts & Steven Levithan.

Explore, test:

- {Rubular}[https://rubular.com/]: interactive online editor.
