Regexp

        # Regexp < Object

(from ruby core)
---

A Regexp holds a regular expression, used to match a pattern against
strings. Regexps are created using the `/.../` and `%r{...}` literals,
and by the Regexp::new constructor.

You can create a Regexp object explicitly with:

*   A [regexp
    literal](doc/syntax/literals_rdoc.html#label-Regexp+Literals).


Regular expressions (*regexp*s) are patterns which describe the contents
of a string. They're used for testing whether a string contains a given
pattern, or extracting the portions that match. They are created with
the `/`*pat*`/` and `%r{`*pat*`}` literals or the `Regexp.new`
constructor.

A regexp is usually delimited with forward slashes (`/`). For example:

    /hay/ =~ 'haystack'   #=> 0
    /y/.match('haystack') #=> #<MatchData "y">

If a string contains the pattern it is said to *match*. A literal string
matches itself.

Here 'haystack' does not contain the pattern 'needle', so it doesn't
match:

    /needle/.match('haystack') #=> nil

Here 'haystack' contains the pattern 'hay', so it matches:

    /hay/.match('haystack')    #=> #<MatchData "hay">

Specifically, `/st/` requires that the string contains the letter *s*
followed by the letter *t*, so it matches *haystack*, also.

## `=~` and Regexp#match

Pattern matching may be achieved by using `=~` operator or Regexp#match
method.

### `=~` operator

`=~` is Ruby's basic pattern-matching operator.  When one operand is a
regular expression and the other is a string then the regular expression
is used as a pattern to match against the string.  (This operator is
equivalently defined by Regexp and String so the order of String and
Regexp do not matter. Other classes may have different implementations
of `=~`.)  If a match is found, the operator returns index of first
match in string, otherwise it returns `nil`.

    /hay/ =~ 'haystack'   #=> 0
    'haystack' =~ /hay/   #=> 0
    /a/   =~ 'haystack'   #=> 1
    /u/   =~ 'haystack'   #=> nil

Using `=~` operator with a String and Regexp the `$~` global variable is
set after a successful match.  `$~` holds a MatchData object.
Regexp.last_match is equivalent to `$~`.

### Regexp#match method

The #match method returns a MatchData object:

    /st/.match('haystack')   #=> #<MatchData "st">

## Metacharacters and Escapes

The following are *metacharacters* `(`, `)`, `[`, `]`, `{`, `}`, `.`,
`?`, `+`, `*`. They have a specific meaning when appearing in a pattern.
To match them literally they must be backslash-escaped. To match a
backslash literally, backslash-escape it: `\\\`.

    /1 \+ 2 = 3\?/.match('Does 1 + 2 = 3?') #=> #<MatchData "1 + 2 = 3?">
    /a\\\\b/.match('a\\\\b')                    #=> #<MatchData "a\\b">

Patterns behave like double-quoted strings and can contain the same
backslash escapes (the meaning of `\s` is different, however, see
[below](#label-Character+Classes)).

    /\s\u{6771 4eac 90fd}/.match("Go to 東京都")
        #=> #<MatchData " 東京都">

Arbitrary Ruby expressions can be embedded into patterns with the
`#{...}` construct.

    place = "東京都"
    /#{place}/.match("Go to 東京都")
        #=> #<MatchData "東京都">

## Character Classes

A *character class* is delimited with square brackets (`[`, `]`) and
lists characters that may appear at that point in the match. `/[ab]/`
means *a* or *b*, as opposed to `/ab/` which means *a* followed by *b*.

    /W[aeiou]rd/.match("Word") #=> #<MatchData "Word">

Within a character class the hyphen (`-`) is a metacharacter denoting an
inclusive range of characters. `[abcd]` is equivalent to `[a-d]`. A
range can be followed by another range, so `[abcdwxyz]` is equivalent to
`[a-dw-z]`. The order in which ranges or individual characters appear
inside a character class is irrelevant.

    /[0-9a-f]/.match('9f') #=> #<MatchData "9">
    /[9f]/.match('9f')     #=> #<MatchData "9">

If the first character of a character class is a caret (`^`) the class
is inverted: it matches any character *except* those named.

    /[^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]/

The following metacharacters also behave like character classes:

*   `/./` - Any character except a newline.
*   `/./m` - Any character (the `m` modifier enables multiline mode)
*   `/\w/` - A word character (`[a-zA-Z0-9_]`)
*   `/\W/` - A non-word character (`[^a-zA-Z0-9_]`). Please take a look
    at [Bug #4044](https://bugs.ruby-lang.org/issues/4044) if using
    `/\W/` with the `/i` modifier.
*   `/\d/` - A digit character (`[0-9]`)
*   `/\D/` - A non-digit character (`[^0-9]`)
*   `/\h/` - A hexdigit character (`[0-9a-fA-F]`)
*   `/\H/` - A non-hexdigit character (`[^0-9a-fA-F]`)
*   `/\s/` - A whitespace character: `/[ \t\r\n\f\v]/`
*   `/\S/` - A non-whitespace character: `/[^ \t\r\n\f\v]/`
*   `/\R/` - A linebreak: `\n`, `\v`, `\f`, `\r` `\u0085` (NEXT LINE),
    `\u2028` (LINE SEPARATOR), `\u2029` (PARAGRAPH SEPARATOR) or `\r\n`.


POSIX *bracket expressions* are also similar to character classes. They
provide a portable alternative to the above, with the added benefit that
they encompass non-ASCII characters. For instance, `/\d/` matches only
the ASCII decimal digits (0-9); whereas `/[[:digit:]]/` matches any
character in the Unicode *Nd* category.

*   `/[[:alnum:]]/` - Alphabetic and numeric character
*   `/[[:alpha:]]/` - Alphabetic character
*   `/[[:blank:]]/` - Space or tab
*   `/[[:cntrl:]]/` - Control character
*   `/[[:digit:]]/` - Digit
*   `/[[:graph:]]/` - Non-blank character (excludes spaces, control
    characters, and similar)
*   `/[[:lower:]]/` - Lowercase alphabetical character
*   `/[[:print:]]/` - Like [:graph:], but includes the space character
*   `/[[:punct:]]/` - Punctuation character
*   `/[[:space:]]/` - Whitespace character (`[:blank:]`, newline,
    carriage return, etc.)
*   `/[[:upper:]]/` - Uppercase alphabetical
*   `/[[:xdigit:]]/` - Digit allowed in a hexadecimal number (i.e.,
    0-9a-fA-F)


Ruby also supports the following non-POSIX character classes:

*   `/[[:word:]]/` - A character in one of the following Unicode general
    categories *Letter*, *Mark*, *Number*, *Connector_Punctuation*
*   `/[[:ascii:]]/` - A character in the ASCII character set

        # U+06F2 is "EXTENDED ARABIC-INDIC DIGIT TWO"
        /[[:digit:]]/.match("\u06F2")    #=> #<MatchData "\u{06F2}">
        /[[:upper:]][[:lower:]]/.match("Hello") #=> #<MatchData "He">
        /[[:xdigit:]][[:xdigit:]]/.match("A6")  #=> #<MatchData "A6">


## Repetition

The constructs described so far match a single character. They can be
followed by a repetition metacharacter to specify how many times they
need to occur. Such metacharacters are called *quantifiers*.

*   `*` - Zero or more times
*   `+` - One or more times
*   `?` - Zero or one times (optional)
*   `{`*n*`}` - Exactly *n* times
*   `{`*n*`,}` - *n* or more times
*   `{,`*m*`}` - *m* or less times
*   `{`*n*`,`*m*`}` - At least *n* and at most *m* times


At least one uppercase character ('H'), at least one lowercase character
('e'), two 'l' characters, then one 'o':

    "Hello".match(/[[:upper:]]+[[:lower:]]+l{2}o/) #=> #<MatchData "Hello">

### Greedy match

Repetition is *greedy* by default: as many occurrences as possible are
matched while still allowing the overall match to succeed. By contrast,
*lazy* matching makes the minimal amount of matches necessary for
overall success. Most greedy metacharacters can be made lazy by
following them with `?`. For the `{n}` pattern, because it specifies an
exact number of characters to match and not a variable number of
characters, the `?` metacharacter instead makes the repeated pattern
optional.

Both patterns below match the string. The first uses a greedy quantifier
so '.+' matches '<a><b>'; the second uses a lazy quantifier so '.+?'
matches '<a>':

    /<.+>/.match("<a><b>")  #=> #<MatchData "<a><b>">
    /<.+?>/.match("<a><b>") #=> #<MatchData "<a>">

### Possessive match

A quantifier followed by `+` matches *possessively*: once it has matched
it does not backtrack. They behave like greedy quantifiers, but having
matched they refuse to "give up" their match even if this jeopardises
the overall match.

    /<.*><.+>/.match("<a><b>") #=> #<MatchData "<a><b>">
    /<.*+><.+>/.match("<a><b>") #=> nil
    /<.*><.++>/.match("<a><b>") #=> nil

## Capturing

Parentheses can be used for *capturing*. The text enclosed by the *n*th
group of parentheses can be subsequently referred to with *n*. Within a
pattern use the *backreference* `\n` (e.g. `\1`); outside of the pattern
use `MatchData[n]` (e.g. `MatchData[1]`).

In this example, `'at'` is captured by the first group of parentheses,
then referred to later with `\1`:

    /[csh](..) [csh]\1 in/.match("The cat sat in the hat")
        #=> #<MatchData "cat sat in" 1:"at">

Regexp#match returns a MatchData object which makes the captured text
available with its #[] method:

    /[csh](..) [csh]\1 in/.match("The cat sat in the hat")[1] #=> 'at'

While Ruby supports an arbitrary number of numbered captured groups,
only groups 1-9 are supported using the `\n` backreference syntax.

Ruby also supports `\0` as a special backreference, which references the
entire matched string.  This is also available at `MatchData[0]`.  Note
that the `\0` backreference cannot be used inside the regexp, as
backreferences can only be used after the end of the capture group, and
the `\0` backreference uses the implicit capture group of the entire
match.  However, you can use this backreference when doing substitution:

    "The cat sat in the hat".gsub(/[csh]at/, '\0s')
      # => "The cats sats in the hats"

### Named captures

Capture groups can be referred to by name when defined with the
`(?<`*name*`>)` or `(?'`*name*`')` constructs.

    /\$(?<dollars>\d+)\.(?<cents>\d+)/.match("$3.67")
        #=> #<MatchData "$3.67" dollars:"3" cents:"67">
    /\$(?<dollars>\d+)\.(?<cents>\d+)/.match("$3.67")[:dollars] #=> "3"

Named groups can be backreferenced with `\k<`*name*`>`, where *name* is
the group name.

    /(?<vowel>[aeiou]).\k<vowel>.\k<vowel>/.match('ototomy')
        #=> #<MatchData "ototo" vowel:"o">

**Note**: A regexp can't use named backreferences and numbered
backreferences simultaneously. Also, if a named capture is used in a
regexp, then parentheses used for grouping which would otherwise result
in a unnamed capture are treated as non-capturing.

    /(\w)(\w)/.match("ab").captures # => ["a", "b"]
    /(\w)(\w)/.match("ab").named_captures # => {}

    /(?<c>\w)(\w)/.match("ab").captures # => ["a"]
    /(?<c>\w)(\w)/.match("ab").named_captures # => {"c"=>"a"}

When named capture groups are used with a literal regexp on the
left-hand side of an expression and the `=~` operator, the captured text
is also assigned to local variables with corresponding names.

    /\$(?<dollars>\d+)\.(?<cents>\d+)/ =~ "$3.67" #=> 0
    dollars #=> "3"

## Grouping

Parentheses also *group* the terms they enclose, allowing them to be
quantified as one *atomic* whole.

The pattern below matches a vowel followed by 2 word characters:

    /[aeiou]\w{2}/.match("Caenorhabditis elegans") #=> #<MatchData "aen">

Whereas the following pattern matches a vowel followed by a word
character, twice, i.e. `[aeiou]\w[aeiou]\w`: 'enor'.

    /([aeiou]\w){2}/.match("Caenorhabditis elegans")
        #=> #<MatchData "enor" 1:"or">

The `(?:`...`)` construct provides grouping without capturing. That is,
it combines the terms it contains into an atomic whole without creating
a backreference. This benefits performance at the slight expense of
readability.

The first group of parentheses captures 'n' and the second 'ti'. The
second group is referred to later with the backreference `\2`:

    /I(n)ves(ti)ga\2ons/.match("Investigations")
        #=> #<MatchData "Investigations" 1:"n" 2:"ti">

The first group of parentheses is now made non-capturing with '?:', so
it still matches 'n', but doesn't create the backreference. Thus, the
backreference `\1` now refers to 'ti'.

    /I(?:n)ves(ti)ga\1ons/.match("Investigations")
        #=> #<MatchData "Investigations" 1:"ti">

### Atomic Grouping

Grouping can be made *atomic* with `(?>`*pat*`)`. This causes the
subexpression *pat* to be matched independently of the rest of the
expression such that what it matches becomes fixed for the remainder of
the match, unless the entire subexpression must be abandoned and
subsequently revisited. In this way *pat* is treated as a non-divisible
whole. Atomic grouping is typically used to optimise patterns so as to
prevent the regular expression engine from backtracking needlessly.

The `"` in the pattern below matches the first character of the string,
then `.*` matches *Quote"*. This causes the overall match to fail, so
the text matched by `.*` is backtracked by one position, which leaves
the final character of the string available to match `"`

    /".*"/.match('"Quote"')     #=> #<MatchData "\"Quote\"">

If `.*` is grouped atomically, it refuses to backtrack *Quote"*, even
though this means that the overall match fails

    /"(?>.*)"/.match('"Quote"') #=> nil

## Subexpression Calls

The `\g<`*name*`>` syntax matches the previous subexpression named
*name*, which can be a group name or number, again. This differs from
backreferences in that it re-executes the group rather than simply
trying to re-match the same text.

This pattern matches a *(* character and assigns it to the `paren`
group, tries to call that the `paren` sub-expression again but fails,
then matches a literal *)*:

    /\A(?<paren>\(\g<paren>*\))*\z/ =~ '()'

    /\A(?<paren>\(\g<paren>*\))*\z/ =~ '(())' #=> 0
    # ^1
    #      ^2
    #           ^3
    #                 ^4
    #      ^5
    #           ^6
    #                      ^7
    #                       ^8
    #                       ^9
    #                           ^10

1.  Matches at the beginning of the string, i.e. before the first
    character.
2.  Enters a named capture group called `paren`
3.  Matches a literal *(*, 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 a literal *(*, the second character in the string
7.  Try to call `paren` a third time, but fail because doing so would
    prevent an overall successful match
8.  Match a literal *)*, the third character in the string. Marks the
    end of the second recursive call
9.  Match a literal *)*, the fourth character in the string
10. Match the end of the string


## Alternation

The vertical bar metacharacter (`|`) combines several expressions into a
single one that matches any of the expressions. Each expression is an
*alternative*.

    /\w(and|or)\w/.match("Feliformia") #=> #<MatchData "form" 1:"or">
    /\w(and|or)\w/.match("furandi")    #=> #<MatchData "randi" 1:"and">
    /\w(and|or)\w/.match("dissemblance") #=> nil

## Character Properties

The `\p{}` construct matches characters with the named property, much
like POSIX bracket classes.

*   `/\p{Alnum}/` - Alphabetic and numeric character
*   `/\p{Alpha}/` - Alphabetic character
*   `/\p{Blank}/` - Space or tab
*   `/\p{Cntrl}/` - Control character
*   `/\p{Digit}/` - Digit
*   `/\p{Graph}/` - Non-blank character (excludes spaces, control
    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)
*   `/\p{Word}/` - A member of one of the following Unicode general
    category *Letter*, *Mark*, *Number*, *Connector_Punctuation*
*   `/\p{ASCII}/` - A character in the ASCII character set
*   `/\p{Any}/` - Any Unicode character (including unassigned
    characters)
*   `/\p{Assigned}/` - An assigned character


A Unicode character's *General Category* value can also be matched with
`\p{`*Ab*`}` where *Ab* is the category's abbreviation as described
below:

*   `/\p{L}/` - 'Letter'
*   `/\p{Ll}/` - 'Letter: Lowercase'
*   `/\p{Lm}/` - 'Letter: Mark'
*   `/\p{Lo}/` - 'Letter: Other'
*   `/\p{Lt}/` - 'Letter: Titlecase'
*   `/\p{Lu}/` - 'Letter: Uppercase
*   `/\p{Lo}/` - 'Letter: Other'
*   `/\p{M}/` - 'Mark'
*   `/\p{Mn}/` - 'Mark: Nonspacing'
*   `/\p{Mc}/` - 'Mark: Spacing Combining'
*   `/\p{Me}/` - 'Mark: Enclosing'
*   `/\p{N}/` - 'Number'
*   `/\p{Nd}/` - 'Number: Decimal Digit'
*   `/\p{Nl}/` - 'Number: Letter'
*   `/\p{No}/` - 'Number: Other'
*   `/\p{P}/` - 'Punctuation'
*   `/\p{Pc}/` - 'Punctuation: Connector'
*   `/\p{Pd}/` - 'Punctuation: Dash'
*   `/\p{Ps}/` - 'Punctuation: Open'
*   `/\p{Pe}/` - 'Punctuation: Close'
*   `/\p{Pi}/` - 'Punctuation: Initial Quote'
*   `/\p{Pf}/` - 'Punctuation: Final Quote'
*   `/\p{Po}/` - 'Punctuation: Other'
*   `/\p{S}/` - 'Symbol'
*   `/\p{Sm}/` - 'Symbol: Math'
*   `/\p{Sc}/` - 'Symbol: Currency'
*   `/\p{Sc}/` - 'Symbol: Currency'
*   `/\p{Sk}/` - 'Symbol: Modifier'
*   `/\p{So}/` - 'Symbol: Other'
*   `/\p{Z}/` - 'Separator'
*   `/\p{Zs}/` - 'Separator: Space'
*   `/\p{Zl}/` - 'Separator: Line'
*   `/\p{Zp}/` - 'Separator: Paragraph'
*   `/\p{C}/` - 'Other'
*   `/\p{Cc}/` - 'Other: Control'
*   `/\p{Cf}/` - 'Other: Format'
*   `/\p{Cn}/` - 'Other: Not Assigned'
*   `/\p{Co}/` - 'Other: Private Use'
*   `/\p{Cs}/` - 'Other: Surrogate'


Lastly, `\p{}` matches a character's Unicode *script*. The following
scripts are supported: *Arabic*, *Armenian*, *Balinese*, *Bengali*,
*Bopomofo*, *Braille*, *Buginese*, *Buhid*, *Canadian_Aboriginal*,
*Carian*, *Cham*, *Cherokee*, *Common*, *Coptic*, *Cuneiform*,
*Cypriot*, *Cyrillic*, *Deseret*, *Devanagari*, *Ethiopic*, *Georgian*,
*Glagolitic*, *Gothic*, *Greek*, *Gujarati*, *Gurmukhi*, *Han*,
*Hangul*, *Hanunoo*, *Hebrew*, *Hiragana*, *Inherited*, *Kannada*,
*Katakana*, *Kayah_Li*, *Kharoshthi*, *Khmer*, *Lao*, *Latin*, *Lepcha*,
*Limbu*, *Linear_B*, *Lycian*, *Lydian*, *Malayalam*, *Mongolian*,
*Myanmar*, *New_Tai_Lue*, *Nko*, *Ogham*, *Ol_Chiki*, *Old_Italic*,
*Old_Persian*, *Oriya*, *Osmanya*, *Phags_Pa*, *Phoenician*, *Rejang*,
*Runic*, *Saurashtra*, *Shavian*, *Sinhala*, *Sundanese*,
*Syloti_Nagri*, *Syriac*, *Tagalog*, *Tagbanwa*, *Tai_Le*, *Tamil*,
*Telugu*, *Thaana*, *Thai*, *Tibetan*, *Tifinagh*, *Ugaritic*, *Vai*,
and *Yi*.

Unicode codepoint U+06E9 is named "ARABIC PLACE OF SAJDAH" and belongs
to the Arabic script:

    /\p{Arabic}/.match("\u06E9") #=> #<MatchData "\u06E9">

All character properties can be inverted by prefixing their name with a
caret (`^`).

Letter 'A' is not in the Unicode Ll (Letter; Lowercase) category, so
this match succeeds:

    /\p{^Ll}/.match("A") #=> #<MatchData "A">

## Anchors

Anchors are metacharacter that match the zero-width positions between
characters, *anchoring* the match to a specific position.

*   `^` - Matches beginning of line
*   `$` - Matches end of line
*   `\A` - Matches beginning of string.
*   `\Z` - Matches end of string. If string ends with a newline, it
    matches just before newline
*   `\z` - Matches end of string
*   `\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

*   `\b` - Matches word boundaries when outside brackets; backspace
    (0x08) when inside brackets
*   `\B` - Matches non-word boundaries
*   `(?=`*pat*`)` - *Positive lookahead* assertion: ensures that the
    following characters match *pat*, but doesn't include those
    characters in the matched text
*   `(?!`*pat*`)` - *Negative lookahead* assertion: ensures that the
    following characters do not match *pat*, but doesn't include those
    characters in the matched text
*   `(?<=`*pat*`)` - *Positive lookbehind* assertion: ensures that the
    preceding characters match *pat*, but doesn't include those
    characters in the matched text
*   `(?<!`*pat*`)` - *Negative lookbehind* assertion: ensures that the
    preceding characters do not match *pat*, but doesn't include those
    characters in the matched text
*   `\K` - Uses an positive lookbehind of the content preceding `\K` in
    the regexp.  For example, the following two regexps are almost
    equivalent:

        /ab\Kc/
        /(?<=ab)c/

    As are the following two regexps:

        /(a)\K(b)\Kc/
        /(?<=(?<=(a))(b))c/


If a pattern isn't anchored it can begin at any point in the string:

    /real/.match("surrealist") #=> #<MatchData "real">

Anchoring the pattern to the beginning of the string forces the match to
start there. 'real' doesn't occur at the beginning of the string, so now
the match fails:

    /\Areal/.match("surrealist") #=> nil

The match below fails because although 'Demand' contains 'and', the
pattern does not occur at a word boundary.

    /\band/.match("Demand")

Whereas in the following example 'and' has been anchored to a non-word
boundary so instead of matching the first 'and' it matches from the
fourth letter of 'demand' instead:

    /\Band.+/.match("Supply and demand curve") #=> #<MatchData "and curve">

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 favours the <b>bold</b>")
        #=> #<MatchData "bold">

## Options

The end delimiter for a regexp can be followed by one or more
single-letter options which control how the pattern can match.

*   `/pat/i` - Ignore case
*   `/pat/m` - Treat a newline as a character matched by `.`
*   `/pat/x` - Ignore whitespace and comments in the pattern
*   `/pat/o` - Perform `#{}` interpolation only once


`i`, `m`, and `x` can also be applied on the subexpression level with
the `(?`*on*`-`*off*`)` construct, which enables options *on*, and
disables options *off* for the expression enclosed by the parentheses:

    /a(?i:b)c/.match('aBc')   #=> #<MatchData "aBc">
    /a(?-i:b)c/i.match('ABC') #=> nil

Additionally, these options can also be toggled for the remainder of the
pattern:

    /a(?i)bc/.match('abC') #=> #<MatchData "abC">

Options may also be used with `Regexp.new`:

    Regexp.new("abc", Regexp::IGNORECASE)                     #=> /abc/i
    Regexp.new("abc", Regexp::MULTILINE)                      #=> /abc/m
    Regexp.new("abc # Comment", Regexp::EXTENDED)             #=> /abc # Comment/x
    Regexp.new("abc", Regexp::IGNORECASE | Regexp::MULTILINE) #=> /abc/mi

## Free-Spacing Mode and Comments

As mentioned above, the `x` option enables *free-spacing* mode. Literal
white space inside the pattern is ignored, and the octothorpe (`#`)
character introduces a comment until the end of the line. This allows
the components of the pattern to be organized in a potentially more
readable fashion.

A contrived pattern to match a number with optional decimal places:

    float_pat = /\A
        [[:digit:]]+ # 1 or more digits before the decimal point
        (\.          # Decimal point
            [[:digit:]]+ # 1 or more digits after the decimal point
        )? # The decimal point and following digits are optional
    \Z/x
    float_pat.match('3.14') #=> #<MatchData "3.14" 1:".14">

There are a number of strategies for matching whitespace:

*   Use a pattern such as `\s` or `\p{Space}`.
*   Use escaped whitespace such as `\ `, i.e. a space preceded by a
    backslash.
*   Use a character class such as `[ ]`.


Comments can be included in a non-`x` pattern with the `(?#`*comment*`)`
construct, where *comment* is arbitrary text ignored by the regexp
engine.

Comments in regexp literals cannot include unescaped terminator
characters.

## Encoding

Regular expressions are assumed to use the source encoding. This can be
overridden with one of the following modifiers.

*   `/`*pat*`/u` - UTF-8
*   `/`*pat*`/e` - EUC-JP
*   `/`*pat*`/s` - Windows-31J
*   `/`*pat*`/n` - ASCII-8BIT


A regexp can be matched against a string when they either share an
encoding, or the regexp's encoding is *US-ASCII* and the string's
encoding is ASCII-compatible.

If a match between incompatible encodings is attempted an
`Encoding::CompatibilityError` exception is raised.

The `Regexp#fixed_encoding?` predicate indicates whether the regexp has
a *fixed* encoding, that is one incompatible with ASCII. A regexp's
encoding can be explicitly fixed by supplying `Regexp::FIXEDENCODING` as
the second argument of `Regexp.new`:

    r = Regexp.new("a".force_encoding("iso-8859-1"),Regexp::FIXEDENCODING)
    r =~ "a\u3042"
       # raises Encoding::CompatibilityError: incompatible encoding regexp match
       #         (ISO-8859-1 regexp with UTF-8 string)

## Special global variables

Pattern matching sets some global variables :
*   `$~` is equivalent to Regexp.last_match;
*   `$&` contains the complete matched text;
*   `$`` contains string before match;
*   `$'` contains string after match;
*   `$1`, `$2` and so on contain text matching first, second, etc
    capture group;
*   `$+` contains last capture group.


Example:

    m = /s(\w{2}).*(c)/.match('haystack') #=> #<MatchData "stac" 1:"ta" 2:"c">
    $~                                    #=> #<MatchData "stac" 1:"ta" 2:"c">
    Regexp.last_match                     #=> #<MatchData "stac" 1:"ta" 2:"c">

    $&      #=> "stac"
            # same as m[0]
    $`      #=> "hay"
            # same as m.pre_match
    $'      #=> "k"
            # same as m.post_match
    $1      #=> "ta"
            # same as m[1]
    $2      #=> "c"
            # same as m[2]
    $3      #=> nil
            # no third group in pattern
    $+      #=> "c"
            # same as m[-1]

These global variables are thread-local and method-local variables.

## Performance

Certain pathological combinations of constructs can lead to abysmally
bad performance.

Consider a string of 25 *a*s, a *d*, 4 *a*s, and a *c*.

    s = 'a' * 25 + 'd' + 'a' * 4 + 'c'
    #=> "aaaaaaaaaaaaaaaaaaaaaaaaadaaaac"

The following patterns match instantly as you would expect:

    /(b|a)/ =~ s #=> 0
    /(b|a+)/ =~ s #=> 0
    /(b|a+)*/ =~ s #=> 0

However, the following pattern takes appreciably longer:

    /(b|a+)*c/ =~ s #=> 26

This happens because an atom in the regexp is quantified by both an
immediate `+` and an enclosing `*` with nothing to differentiate which
is in control of any particular character. The nondeterminism that
results produces super-linear performance. (Consult *Mastering Regular
Expressions* (3rd ed.), pp 222, by *Jeffery Friedl*, for an in-depth
analysis). This particular case can be fixed by use of atomic grouping,
which prevents the unnecessary backtracking:

    (start = Time.now) && /(b|a+)*c/ =~ s && (Time.now - start)
       #=> 24.702736882
    (start = Time.now) && /(?>b|a+)*c/ =~ s && (Time.now - start)
       #=> 0.000166571

A similar case is typified by the following example, which takes
approximately 60 seconds to execute for me:

Match a string of 29 *a*s against a pattern of 29 optional *a*s followed
by 29 mandatory *a*s:

    Regexp.new('a?' * 29 + 'a' * 29) =~ 'a' * 29

The 29 optional *a*s match the string, but this prevents the 29
mandatory *a*s that follow from matching. Ruby must then backtrack
repeatedly so as to satisfy as many of the optional matches as it can
while still matching the mandatory 29. It is plain to us that none of
the optional matches can succeed, but this fact unfortunately eludes
Ruby.

The best way to improve performance is to significantly reduce the
amount of backtracking needed.  For this case, instead of individually
matching 29 optional *a*s, a range of optional *a*s can be matched all
at once with *a{0,29}*:

    Regexp.new('a{0,29}' + 'a' * 29) =~ 'a' * 29
---
# Constants:

EXTENDED
:   see Regexp.options and Regexp.new

FIXEDENCODING
:   see Regexp.options and Regexp.new

IGNORECASE
:   see Regexp.options and Regexp.new

MULTILINE
:   see Regexp.options and Regexp.new

NOENCODING
:   see Regexp.options and Regexp.new



# Class methods:

    compile
    escape
    json_create
    last_match
    new
    quote
    try_convert
    union

# Instance methods:

    ==
    ===
    =~
    as_json
    casefold?
    encoding
    eql?
    fixed_encoding?
    hash
    inspect
    match
    match?
    named_captures
    names
    options
    source
    to_json
    to_s
    ~