# class Regexp

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.

Regular expressions (regexps) 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).

    /\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 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">

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>">

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.

## Capturing¶ ↑

Parentheses can be used for capturing. The text enclosed by the n<sup>th</sup> group of parentheses can be subsequently referred to with n. Within a pattern use the backreference `\n`; outside of the pattern use `MatchData[n]`.

'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'

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 two expressions into a single one that matches either 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

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 as, a d, 4 as, 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 as against a pattern of 29 optional as followed by 29 mandatory as:

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

The 29 optional as match the string, but this prevents the 29 mandatory as 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 as, a range of optional as 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`

### Public Class Methods

compile(*args)

Alias for `Regexp.new`

escape(str) → string click to toggle source

Escapes any characters that would have special meaning in a regular expression. Returns a new escaped string with the same or compatible encoding. For any string, `Regexp.new(Regexp.escape(str))=~str` will be true.

    Regexp.escape('\*?{}.')   #=> \\\*\?\{\}\.

    static VALUE
    rb_reg_s_quote(VALUE c, VALUE str)
    {
        return rb_reg_quote(reg_operand(str, TRUE));
    }

json_create(object) click to toggle source

Deserializes `JSON` string by constructing new `Regexp` object with source `s` (`Regexp` or `String`) and options `o` serialized by `to_json`

    # File ext/json/lib/json/add/regexp.rb, line 11
    def self.json_create(object)
      new(object['s'], object['o'])
    end

last_match → matchdata click to toggle source
last_match(n) → str

The first form returns the `MatchData` object generated by the last successful pattern match. Equivalent to reading the special global variable `$~` (see Special global variables in `Regexp` for details).

The second form returns the nth field in this `MatchData` object. n can be a string or symbol to reference a named capture.

Note that the `last_match` is local to the thread and method scope of the method that did the pattern match.

    /c(.)t/ =~ 'cat'        #=> 0
    Regexp.last_match       #=> #<MatchData "cat" 1:"a">
    Regexp.last_match(0)    #=> "cat"
    Regexp.last_match(1)    #=> "a"
    Regexp.last_match(2)    #=> nil

    /(?<lhs>\w+)\s*=\s*(?<rhs>\w+)/ =~ "var = val"
    Regexp.last_match       #=> #<MatchData "var = val" lhs:"var" rhs:"val">
    Regexp.last_match(:lhs) #=> "var"
    Regexp.last_match(:rhs) #=> "val"

    static VALUE
    rb_reg_s_last_match(int argc, VALUE *argv, VALUE _)
    {
        if (rb_check_arity(argc, 0, 1) == 1) {
            VALUE match = rb_backref_get();
            int n;
            if (NIL_P(match)) return Qnil;
            n = match_backref_number(match, argv[0]);
            return rb_reg_nth_match(n, match);
        }
        return match_getter();
    }

new(string, [options]) → regexp click to toggle source
new(regexp) → regexp
compile(string, [options]) → regexp
compile(regexp) → regexp

Constructs a new regular expression from `pattern`, which can be either a `String` or a `Regexp` (in which case that regexp's options are propagated), and new options may not be specified (a change as of Ruby 1.8).

If `options` is an `Integer`, it should be one or more of the constants `Regexp::EXTENDED`, `Regexp::IGNORECASE`, and `Regexp::MULTILINE`, or-ed together. Otherwise, if `options` is not `nil` or `false`, the regexp will be case insensitive.

    r1 = Regexp.new('^a-z+:\\s+\w+') #=> /^a-z+:\s+\w+/
    r2 = Regexp.new('cat', true)     #=> /cat/i
    r3 = Regexp.new(r2)              #=> /cat/i
    r4 = Regexp.new('dog', Regexp::EXTENDED | Regexp::IGNORECASE) #=> /dog/ix

    static VALUE
    rb_reg_initialize_m(int argc, VALUE *argv, VALUE self)
    {
        int flags = 0;
        VALUE str;
        rb_encoding *enc = 0;

        rb_check_arity(argc, 1, 3);
        if (RB_TYPE_P(argv[0], T_REGEXP)) {
            VALUE re = argv[0];

            if (argc > 1) {
                rb_warn("flags ignored");
            }
            rb_reg_check(re);
            flags = rb_reg_options(re);
            str = RREGEXP_SRC(re);
        }
        else {
            if (argc >= 2) {
                if (FIXNUM_P(argv[1])) flags = FIX2INT(argv[1]);
                else if (RTEST(argv[1])) flags = ONIG_OPTION_IGNORECASE;
            }
            if (argc == 3 && !NIL_P(argv[2])) {
                char *kcode = StringValuePtr(argv[2]);
                if (kcode[0] == 'n' || kcode[0] == 'N') {
                    enc = rb_ascii8bit_encoding();
                    flags |= ARG_ENCODING_NONE;
                }
                else {
                    rb_warn("encoding option is ignored - %s", kcode);
                }
            }
            str = StringValue(argv[0]);
        }
        if (enc && rb_enc_get(str) != enc)
            rb_reg_init_str_enc(self, str, enc, flags);
        else
            rb_reg_init_str(self, str, flags);
        return self;
    }

quote(str) → string click to toggle source

Escapes any characters that would have special meaning in a regular expression. Returns a new escaped string with the same or compatible encoding. For any string, `Regexp.new(Regexp.escape(str))=~str` will be true.

    Regexp.escape('\*?{}.')   #=> \\\*\?\{\}\.

    static VALUE
    rb_reg_s_quote(VALUE c, VALUE str)
    {
        return rb_reg_quote(reg_operand(str, TRUE));
    }

try_convert(obj) → re or nil click to toggle source

Try to convert obj into a `Regexp`, using to_regexp method. Returns converted regexp or nil if obj cannot be converted for any reason.

    Regexp.try_convert(/re/)         #=> /re/
    Regexp.try_convert("re")         #=> nil

    o = Object.new
    Regexp.try_convert(o)            #=> nil
    def o.to_regexp() /foo/ end
    Regexp.try_convert(o)            #=> /foo/

    static VALUE
    rb_reg_s_try_convert(VALUE dummy, VALUE re)
    {
        return rb_check_regexp_type(re);
    }

union(pat1, pat2, ...) → new_regexp click to toggle source
union(pats_ary) → new_regexp

Return a `Regexp` object that is the union of the given patterns, i.e., will match any of its parts. The patterns can be `Regexp` objects, in which case their options will be preserved, or Strings. If no patterns are given, returns `/(?!)/`. The behavior is unspecified if any given pattern contains capture.

    Regexp.union                         #=> /(?!)/
    Regexp.union("penzance")             #=> /penzance/
    Regexp.union("a+b*c")                #=> /a\+b\*c/
    Regexp.union("skiing", "sledding")   #=> /skiing|sledding/
    Regexp.union(["skiing", "sledding"]) #=> /skiing|sledding/
    Regexp.union(/dogs/, /cats/i)        #=> /(?-mix:dogs)|(?i-mx:cats)/

Note: the arguments for `::union` will try to be converted into a regular expression literal via to_regexp.

    static VALUE
    rb_reg_s_union_m(VALUE self, VALUE args)
    {
        VALUE v;
        if (RARRAY_LEN(args) == 1 &&
            !NIL_P(v = rb_check_array_type(rb_ary_entry(args, 0)))) {
            return rb_reg_s_union(self, v);
        }
        return rb_reg_s_union(self, args);
    }

### Public Instance Methods

rxp == other_rxp → true or false click to toggle source

Equality—Two regexps are equal if their patterns are identical, they have the same character set code, and their `casefold?` values are the same.

    /abc/  == /abc/x   #=> false
    /abc/  == /abc/i   #=> false
    /abc/  == /abc/u   #=> false
    /abc/u == /abc/n   #=> false

    static VALUE
    rb_reg_equal(VALUE re1, VALUE re2)
    {
        if (re1 == re2) return Qtrue;
        if (!RB_TYPE_P(re2, T_REGEXP)) return Qfalse;
        rb_reg_check(re1); rb_reg_check(re2);
        if (FL_TEST(re1, KCODE_FIXED) != FL_TEST(re2, KCODE_FIXED)) return Qfalse;
        if (RREGEXP_PTR(re1)->options != RREGEXP_PTR(re2)->options) return Qfalse;
        if (RREGEXP_SRC_LEN(re1) != RREGEXP_SRC_LEN(re2)) return Qfalse;
        if (ENCODING_GET(re1) != ENCODING_GET(re2)) return Qfalse;
        if (memcmp(RREGEXP_SRC_PTR(re1), RREGEXP_SRC_PTR(re2), RREGEXP_SRC_LEN(re1)) == 0) {
            return Qtrue;
        }
        return Qfalse;
    }

rxp === str → true or false click to toggle source

Case Equality—Used in case statements.

    a = "HELLO"
    case a
    when /\A[a-z]*\z/; print "Lower case\n"
    when /\A[A-Z]*\z/; print "Upper case\n"
    else;              print "Mixed case\n"
    end
    #=> "Upper case"

Following a regular expression literal with the `===` operator allows you to compare against a `String`.

    /^[a-z]*$/ === "HELLO" #=> false
    /^[A-Z]*$/ === "HELLO" #=> true

    VALUE
    rb_reg_eqq(VALUE re, VALUE str)
    {
        long start;

        str = reg_operand(str, FALSE);
        if (NIL_P(str)) {
            rb_backref_set(Qnil);
            return Qfalse;
        }
        start = rb_reg_search(re, str, 0, 0);
        if (start < 0) {
            return Qfalse;
        }
        return Qtrue;
    }

rxp =~ str → integer or nil click to toggle source

Match—Matches rxp against str.

    /at/ =~ "input data"   #=> 7
    /ax/ =~ "input data"   #=> nil

If `=~` is used with a regexp literal with named captures, captured strings (or nil) is assigned to local variables named by the capture names.

    /(?<lhs>\w+)\s*=\s*(?<rhs>\w+)/ =~ "  x = y  "
    p lhs    #=> "x"
    p rhs    #=> "y"

If it is not matched, nil is assigned for the variables.

    /(?<lhs>\w+)\s*=\s*(?<rhs>\w+)/ =~ "  x = "
    p lhs    #=> nil
    p rhs    #=> nil

This assignment is implemented in the Ruby parser. The parser detects 'regexp-literal =~ expression' for the assignment. The regexp must be a literal without interpolation and placed at left hand side.

The assignment does not occur if the regexp is not a literal.

    re = /(?<lhs>\w+)\s*=\s*(?<rhs>\w+)/
    re =~ "  x = y  "
    p lhs    # undefined local variable
    p rhs    # undefined local variable

A regexp interpolation, `#{}`, also disables the assignment.

    rhs_pat = /(?<rhs>\w+)/
    /(?<lhs>\w+)\s*=\s*#{rhs_pat}/ =~ "x = y"
    p lhs    # undefined local variable

The assignment does not occur if the regexp is placed at the right hand side.

    "  x = y  " =~ /(?<lhs>\w+)\s*=\s*(?<rhs>\w+)/
    p lhs, rhs # undefined local variable

    VALUE
    rb_reg_match(VALUE re, VALUE str)
    {
        long pos = reg_match_pos(re, &str, 0);
        if (pos < 0) return Qnil;
        pos = rb_str_sublen(str, pos);
        return LONG2FIX(pos);
    }

as_json(*) click to toggle source

Returns a hash, that will be turned into a `JSON` object and represent this object.

    # File ext/json/lib/json/add/regexp.rb, line 17
    def as_json(*)
      {
        JSON.create_id => self.class.name,
        'o'            => options,
        's'            => source,
      }
    end

casefold? → true or false click to toggle source

Returns the value of the case-insensitive flag.

    /a/.casefold?           #=> false
    /a/i.casefold?          #=> true
    /(?i:a)/.casefold?      #=> false

    static VALUE
    rb_reg_casefold_p(VALUE re)
    {
        rb_reg_check(re);
        if (RREGEXP_PTR(re)->options & ONIG_OPTION_IGNORECASE) return Qtrue;
        return Qfalse;
    }

encoding → encoding click to toggle source

Returns the `Encoding` object that represents the encoding of obj.

    VALUE
    rb_obj_encoding(VALUE obj)
    {
        int idx = rb_enc_get_index(obj);
        if (idx < 0) {
    	rb_raise(rb_eTypeError, "unknown encoding");
        }
        return rb_enc_from_encoding_index(idx & ENC_INDEX_MASK);
    }

eql?(other_rxp) → true or false click to toggle source

Equality—Two regexps are equal if their patterns are identical, they have the same character set code, and their `casefold?` values are the same.

    /abc/  == /abc/x   #=> false
    /abc/  == /abc/i   #=> false
    /abc/  == /abc/u   #=> false
    /abc/u == /abc/n   #=> false

    static VALUE
    rb_reg_equal(VALUE re1, VALUE re2)
    {
        if (re1 == re2) return Qtrue;
        if (!RB_TYPE_P(re2, T_REGEXP)) return Qfalse;
        rb_reg_check(re1); rb_reg_check(re2);
        if (FL_TEST(re1, KCODE_FIXED) != FL_TEST(re2, KCODE_FIXED)) return Qfalse;
        if (RREGEXP_PTR(re1)->options != RREGEXP_PTR(re2)->options) return Qfalse;
        if (RREGEXP_SRC_LEN(re1) != RREGEXP_SRC_LEN(re2)) return Qfalse;
        if (ENCODING_GET(re1) != ENCODING_GET(re2)) return Qfalse;
        if (memcmp(RREGEXP_SRC_PTR(re1), RREGEXP_SRC_PTR(re2), RREGEXP_SRC_LEN(re1)) == 0) {
            return Qtrue;
        }
        return Qfalse;
    }

fixed_encoding? → true or false click to toggle source

Returns false if rxp is applicable to a string with any ASCII compatible encoding. Returns true otherwise.

    r = /a/
    r.fixed_encoding?                               #=> false
    r =~ "\u{6666} a"                               #=> 2
    r =~ "\xa1\xa2 a".force_encoding("euc-jp")      #=> 2
    r =~ "abc".force_encoding("euc-jp")             #=> 0

    r = /a/u
    r.fixed_encoding?                               #=> true
    r.encoding                                      #=> #<Encoding:UTF-8>
    r =~ "\u{6666} a"                               #=> 2
    r =~ "\xa1\xa2".force_encoding("euc-jp")        #=> Encoding::CompatibilityError
    r =~ "abc".force_encoding("euc-jp")             #=> 0

    r = /\u{6666}/
    r.fixed_encoding?                               #=> true
    r.encoding                                      #=> #<Encoding:UTF-8>
    r =~ "\u{6666} a"                               #=> 0
    r =~ "\xa1\xa2".force_encoding("euc-jp")        #=> Encoding::CompatibilityError
    r =~ "abc".force_encoding("euc-jp")             #=> nil

    static VALUE
    rb_reg_fixed_encoding_p(VALUE re)
    {
        if (FL_TEST(re, KCODE_FIXED))
            return Qtrue;
        else
            return Qfalse;
    }

hash → integer click to toggle source

Produce a hash based on the text and options of this regular expression.

See also `Object#hash`.

    static VALUE
    rb_reg_hash(VALUE re)
    {
        st_index_t hashval = reg_hash(re);
        return ST2FIX(hashval);
    }

inspect → string click to toggle source

Produce a nicely formatted string-version of rxp. Perhaps surprisingly, `#inspect` actually produces the more natural version of the string than `#to_s`.

    /ab+c/ix.inspect        #=> "/ab+c/ix"

    static VALUE
    rb_reg_inspect(VALUE re)
    {
        if (!RREGEXP_PTR(re) || !RREGEXP_SRC(re) || !RREGEXP_SRC_PTR(re)) {
            return rb_any_to_s(re);
        }
        return rb_reg_desc(RREGEXP_SRC_PTR(re), RREGEXP_SRC_LEN(re), re);
    }

match(str) → matchdata or nil click to toggle source
match(str,pos) → matchdata or nil

Returns a `MatchData` object describing the match, or `nil` if there was no match. This is equivalent to retrieving the value of the special variable `$~` following a normal match. If the second parameter is present, it specifies the position in the string to begin the search.

    /(.)(.)(.)/.match("abc")[2]   #=> "b"
    /(.)(.)/.match("abc", 1)[2]   #=> "c"

If a block is given, invoke the block with `MatchData` if match succeed, so that you can write

    /M(.*)/.match("Matz") do |m|
      puts m[0]
      puts m[1]
    end

instead of

    if m = /M(.*)/.match("Matz")
      puts m[0]
      puts m[1]
    end

The return value is a value from block execution in this case.

    static VALUE
    rb_reg_match_m(int argc, VALUE *argv, VALUE re)
    {
        VALUE result, str, initpos;
        long pos;

        if (rb_scan_args(argc, argv, "11", &str, &initpos) == 2) {
            pos = NUM2LONG(initpos);
        }
        else {
            pos = 0;
        }

        pos = reg_match_pos(re, &str, pos);
        if (pos < 0) {
            rb_backref_set(Qnil);
            return Qnil;
        }
        result = rb_backref_get();
        rb_match_busy(result);
        if (!NIL_P(result) && rb_block_given_p()) {
            return rb_yield(result);
        }
        return result;
    }

match?(str) → true or false click to toggle source
match?(str,pos) → true or false

Returns a `true` or `false` indicates whether the regexp is matched or not without updating $~ and other related variables. If the second parameter is present, it specifies the position in the string to begin the search.

    /R.../.match?("Ruby")    #=> true
    /R.../.match?("Ruby", 1) #=> false
    /P.../.match?("Ruby")    #=> false
    $&                       #=> nil

    static VALUE
    rb_reg_match_m_p(int argc, VALUE *argv, VALUE re)
    {
        long pos = rb_check_arity(argc, 1, 2) > 1 ? NUM2LONG(argv[1]) : 0;
        return rb_reg_match_p(re, argv[0], pos);
    }

named_captures → hash click to toggle source

Returns a hash representing information about named captures of rxp.

A key of the hash is a name of the named captures. A value of the hash is an array which is list of indexes of corresponding named captures.

    /(?<foo>.)(?<bar>.)/.named_captures
    #=> {"foo"=>[1], "bar"=>[2]}

    /(?<foo>.)(?<foo>.)/.named_captures
    #=> {"foo"=>[1, 2]}

If there are no named captures, an empty hash is returned.

    /(.)(.)/.named_captures
    #=> {}

    static VALUE
    rb_reg_named_captures(VALUE re)
    {
        regex_t *reg = (rb_reg_check(re), RREGEXP_PTR(re));
        VALUE hash = rb_hash_new_with_size(onig_number_of_names(reg));
        onig_foreach_name(reg, reg_named_captures_iter, (void*)hash);
        return hash;
    }

names → [name1, name2, ...] click to toggle source

Returns a list of names of captures as an array of strings.

    /(?<foo>.)(?<bar>.)(?<baz>.)/.names
    #=> ["foo", "bar", "baz"]

    /(?<foo>.)(?<foo>.)/.names
    #=> ["foo"]

    /(.)(.)/.names
    #=> []

    static VALUE
    rb_reg_names(VALUE re)
    {
        VALUE ary;
        rb_reg_check(re);
        ary = rb_ary_new_capa(onig_number_of_names(RREGEXP_PTR(re)));
        onig_foreach_name(RREGEXP_PTR(re), reg_names_iter, (void*)ary);
        return ary;
    }

options → integer click to toggle source

Returns the set of bits corresponding to the options used when creating this `Regexp` (see `Regexp::new` for details. Note that additional bits may be set in the returned options: these are used internally by the regular expression code. These extra bits are ignored if the options are passed to `Regexp::new`.

    Regexp::IGNORECASE                  #=> 1
    Regexp::EXTENDED                    #=> 2
    Regexp::MULTILINE                   #=> 4

    /cat/.options                       #=> 0
    /cat/ix.options                     #=> 3
    Regexp.new('cat', true).options     #=> 1
    /\xa1\xa2/e.options                 #=> 16

    r = /cat/ix
    Regexp.new(r.source, r.options)     #=> /cat/ix

    static VALUE
    rb_reg_options_m(VALUE re)
    {
        int options = rb_reg_options(re);
        return INT2NUM(options);
    }

source → str click to toggle source

Returns the original string of the pattern.

    /ab+c/ix.source #=> "ab+c"

Note that escape sequences are retained as is.

    /\x20\+/.source  #=> "\\x20\\+"

    static VALUE
    rb_reg_source(VALUE re)
    {
        VALUE str;

        rb_reg_check(re);
        str = rb_str_dup(RREGEXP_SRC(re));
        return str;
    }

to_json(*args) click to toggle source

Stores class name (`Regexp`) with options `o` and source `s` (`Regexp` or `String`) as `JSON` string

    # File ext/json/lib/json/add/regexp.rb, line 27
    def to_json(*args)
      as_json.to_json(*args)
    end

to_s → str click to toggle source

Returns a string containing the regular expression and its options (using the `(?opts:source)` notation. This string can be fed back in to `Regexp::new` to a regular expression with the same semantics as the original. (However, `Regexp#==` may not return true when comparing the two, as the source of the regular expression itself may differ, as the example shows). `Regexp#inspect` produces a generally more readable version of rxp.

    r1 = /ab+c/ix           #=> /ab+c/ix
    s1 = r1.to_s            #=> "(?ix-m:ab+c)"
    r2 = Regexp.new(s1)     #=> /(?ix-m:ab+c)/
    r1 == r2                #=> false
    r1.source               #=> "ab+c"
    r2.source               #=> "(?ix-m:ab+c)"

    static VALUE
    rb_reg_to_s(VALUE re)
    {
        return rb_reg_str_with_term(re, '/');
    }

~ rxp → integer or nil click to toggle source

Match—Matches rxp against the contents of `$_`. Equivalent to `rxp =~ $_`.

    $_ = "input data"
    ~ /at/   #=> 7

    VALUE
    rb_reg_match2(VALUE re)
    {
        long start;
        VALUE line = rb_lastline_get();

        if (!RB_TYPE_P(line, T_STRING)) {
            rb_backref_set(Qnil);
            return Qnil;
        }

        start = rb_reg_search(re, line, 0, 0);
        if (start < 0) {
            return Qnil;
        }
        start = rb_str_sublen(line, start);
        return LONG2FIX(start);
    }