A Regexpholds 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.newconstructor.

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.

# 'haystack' does not contain the pattern 'needle', so doesn't match.
/needle/.match('haystack') #=> nil
# 'haystack' does contain the pattern 'hay', so it matches
/hay/.match('haystack')    #=> #<MatchData "hay">

Specifically, /st/requires that the string contains the letter sfollowed by the letter t, so it matches haystack, also.

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 that: <tt>\\</tt>.

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

Patterns behave like double-quoted strings so can contain the same backslash escapes.

/\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 classis delimited with square brackets ( [, ]) and lists characters that may appear at that point in the match. /[ab]/means aor b, as opposed to /ab/which means afollowed 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 exceptthose 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 mmodifier enables multiline mode)

  • /\w/- A word character ( [a-zA-Z0-9_])

  • /\W/- A non-word character ( [^a-zA-Z0-9_])

  • /\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]/

  • /\S/- A non-whitespace character: /[^ \t\r\n\f]/

POSIX bracket expressionsare 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 ntimes

  • { n ,}- nor more times

  • {, m }- mor less times

  • { n , m }- At least nand at most mtimes

    # 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 greedyby default: as many occurrences as possible are matched while still allowing the overall match to succeed. By contrast, lazymatching makes the minimal amount of matches necessary for overall success. A greedy metacharacter can be made lazy by following it with ?.

# 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 </tt> n; outside of the pattern use <tt>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 nameis 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.

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 groupthe terms they enclose, allowing them to be quantified as one atomicwhole.

# The pattern below matches a vowel followed by 2 word characters:
# 'aen'
/[aeiou]\w{2}/.match("Caenorhabditis elegans") #=> #<MatchData "aen">
# Whereas the following pattern matches a vowel followed by a word
# character, twice, i.e. <tt>[aeiou]\w[aeiou]\w</tt>: '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 readabilty.

# The 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 atomicwith (?> pat ). This causes the subexpression patto 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 patis treated as a non-divisible whole. Atomic grouping is typically used to optimise patterns so as to prevent the regular expression engine from backtracking needlesly.

# The <tt>"</tt> in the pattern below matches the first character of
# the string, then <tt>.*</tt> matches <i>Quote"</i>. This causes the
# overall match to fail, so the text matched by <tt>.*</tt> is
# backtracked by one position, which leaves the final character of the
# string available to match <tt>"</tt>
      /".*"/.match('"Quote"')     #=> #<MatchData "\"Quote\"">
# If <tt>.*</tt> is grouped atomically, it refuses to backtrack
# <i>Quote"</i>, 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.

# Matches a <i>(</i> character and assigns it to the <tt>paren</tt>
# group, tries to call that the <tt>paren</tt> sub-expression again
# but fails, then matches a literal <i>)</i>.
/\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 parengroup again, i.e. recurses back to the second step

  5. Re-enters the parengroup

  6. Matches a literal (, the second character in the string

  7. Try to call parena 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 Categoryvalue can also be matched with \p{ Ab }where Abis 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, anchoringthe 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 point where last match finished

  • \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 <b></b> 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 xcan also be applied on the subexpression level with the (? on - off ) construct, which enables options on, and disables options offfor the expression enclosed by the parentheses.

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

Free-Spacing Mode and Comments

As mentioned above, the xoption 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 organised 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/
float_pat.match('3.14') #=> #<MatchData "3.14" 1:".14">

Note: To match whitespace in an xpattern use an escape such as \sor \p{Space}.

Comments can be included in a non- xpattern with the (?# comment )construct, where commentis arbitrary text ignored by the regexp engine.

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-ASCIIand the string's encoding is ASCII-compatible.

If a match between incompatible encodings is attempted an Encoding::CompatibilityErrorexception is raised.

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

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

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'
    #=> "aaaaaaaaaaaaaaaaaaaaaaaaadadadadac"

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 #=> 32

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 <i>a</i>s against a pattern of 29 optional
# <i>a</i>s followed by 29 mandatory <i>a</i>s.
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.

One approach for improving performance is to anchor the match to the beginning of the string, thus significantly reducing the amount of backtracking needed.

Regexp.new('\A' 'a?' * 29 + 'a' * 29).match('a' * 29)
    #=> #<MatchData "aaaaaaaaaaaaaaaaaaaaaaaaaaaaa">

Regexp serialization/deserialization

Methods
#
A
C
E
F
H
I
J
L
M
N
O
Q
S
T
U
#
Constants
IGNORECASE = INT2FIX(ONIG_OPTION_IGNORECASE)
 

see #options and ::new

EXTENDED = INT2FIX(ONIG_OPTION_EXTEND)
 

see #options and ::new

MULTILINE = INT2FIX(ONIG_OPTION_MULTILINE)
 

see #options and ::new

FIXEDENCODING = INT2FIX(ARG_ENCODING_FIXED)
 

see #options and ::new

NOENCODING = INT2FIX(ARG_ENCODING_NONE)
 

see #options and ::new

Class Public methods
compile(*args)

Synonym for Regexp.new

Regexp.escape(str) → string Regexp.quote(str) → string

Escapes any characters that would have special meaning in a regular expression. Returns a new escaped string, or self if no characters are escaped. 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)

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/core.rb, line 227
def self.json_create(object)
  new(object['s'], object['o'])
end
Regexp.last_match → matchdata Regexp.last_match(n) → str

The first form returns the MatchDataobject generated by the last successful pattern match. Equivalent to reading the global variable $~. The second form returns the nth field in this MatchDataobject. ncan be a string or symbol to reference a named capture.

Note that the last_matchis 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 nth;

    if (argc > 0 && rb_scan_args(argc, argv, "01", &nth) == 1) {
        VALUE match = rb_backref_get();
        int n;
        if (NIL_P(match)) return Qnil;
        n = match_backref_number(match, nth);
        return rb_reg_nth_match(n, match);
    }
    return match_getter();
}
Regexp.new(string, [options [, lang]]) → regexp Regexp.new(regexp) → regexp Regexp.compile(string, [options [, lang]]) → regexp Regexp.compile(regexp) → regexp

Constructs a new regular expression from pattern, which can be either a Stringor 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 optionsis a Fixnum, it should be one or more of the constants Regexp::EXTENDED, Regexp::IGNORECASE, and Regexp::MULTILINE, or-ed together. Otherwise, if optionsis not nil, the regexp will be case insensitive. When the langparameter is `n' or `N' sets the regexp no encoding.

r1 = Regexp.new('^a-z+:\s+\w+')           #=> /^a-z+:\s+\w+/
r2 = Regexp.new('cat', true)               #=> /cat/i
r3 = Regexp.new('dog', Regexp::EXTENDED)   #=> /dog/x
r4 = Regexp.new(r2)                        #=> /cat/i
static VALUE
rb_reg_initialize_m(int argc, VALUE *argv, VALUE self)
{
    onig_errmsg_buffer err = "";
    int flags = 0;
    VALUE str;
    rb_encoding *enc;
    const char *ptr;
    long len;

    if (argc == 0 || argc > 3) {
        rb_raise(rb_eArgError, "wrong number of arguments (%d for 1..3)", argc);
    }
    if (TYPE(argv[0]) == T_REGEXP) {
        VALUE re = argv[0];

        if (argc > 1) {
            rb_warn("flags ignored");
        }
        rb_reg_check(re);
        flags = rb_reg_options(re);
        ptr = RREGEXP_SRC_PTR(re);
        len = RREGEXP_SRC_LEN(re);
        enc = rb_enc_get(re);
        if (rb_reg_initialize(self, ptr, len, enc, flags, err, NULL, 0)) {
            str = rb_enc_str_new(ptr, len, enc);
            rb_reg_raise_str(str, flags, err);
        }
    }
    else {
        if (argc >= 2) {
            if (FIXNUM_P(argv[1])) flags = FIX2INT(argv[1]);
            else if (RTEST(argv[1])) flags = ONIG_OPTION_IGNORECASE;
        }
        enc = 0;
        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 = argv[0];
        ptr = StringValuePtr(str);
        if (enc
            ? rb_reg_initialize(self, ptr, RSTRING_LEN(str), enc, flags, err, NULL, 0)
            : rb_reg_initialize_str(self, str, flags, err, NULL, 0)) {
            rb_reg_raise_str(str, flags, err);
        }
    }
    return self;
}
Regexp.escape(str) → string Regexp.quote(str) → string

Escapes any characters that would have special meaning in a regular expression. Returns a new escaped string, or self if no characters are escaped. 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));
}
Regexp.try_convert(obj) → re or nil

Try to convert objinto a Regexp, using to_regexp method. Returns converted regexp or nil if objcannot 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);
}
Regexp.union(pat1, pat2, ...) → new_regexp Regexp.union(pats_ary) → new_regexp

Return a Regexpobject 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 patterncontains 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/)        #=> /(?-mix:dogs)|(?i-mx:cats)/
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);
}
Instance Public methods
rxp == other_rxp → true or false rxp.eql?(other_rxp) → true or false

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/   #=> false
/abc/  == /abc/   #=> false
/abc/  == /abc/   #=> false
/abc/ == /abc/   #=> false
static VALUE
rb_reg_equal(VALUE re1, VALUE re2)
{
    if (re1 == re2) return Qtrue;
    if (TYPE(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(re1)->ptr->options != RREGEXP(re2)->ptr->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

Case Equality—Synonym for Regexp#=~used in case statements.

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

produces:

Upper case
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

Match—Matches rxpagainst 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(*)

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

# File ext/json/lib/json/add/core.rb, line 233
def as_json(*)
  {
    JSON.create_id => self.class.name,
    'o'            => options,
    's'            => source,
  }
end
rxp.casefold? → true or false

Returns the value of the case-insensitive flag.

/a/.casefold?           #=> false
/a/.casefold?          #=> true
/(?i:a)/.casefold?      #=> false
static VALUE
rb_reg_casefold_p(VALUE re)
{
    rb_reg_check(re);
    if (RREGEXP(re)->ptr->options & ONIG_OPTION_IGNORECASE) return Qtrue;
    return Qfalse;
}
obj.encoding → encoding

Returns the Encoding object that represents the encoding of obj.

VALUE
rb_obj_encoding(VALUE obj)
{
    rb_encoding *enc = rb_enc_get(obj);
    if (!enc) {
    rb_raise(rb_eTypeError, "unknown encoding");
    }
    return rb_enc_from_encoding(enc);
}
rxp == other_rxp → true or false rxp.eql?(other_rxp) → true or false

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/   #=> false
/abc/  == /abc/   #=> false
/abc/  == /abc/   #=> false
/abc/ == /abc/   #=> false
static VALUE
rb_reg_equal(VALUE re1, VALUE re2)
{
    if (re1 == re2) return Qtrue;
    if (TYPE(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(re1)->ptr->options != RREGEXP(re2)->ptr->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.fixed_encoding? → true or false

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/
r.fixed_encoding?                               #=> true
r.encoding                                      #=> #<Encoding:UTF-8>
r =~ "\u{6666} a"                               #=> 2
r =~ "\xa1\xa2".force_encoding("euc-jp")        #=> ArgumentError
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")        #=> ArgumentError
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;
}
rxp.hash → fixnum

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

static VALUE
rb_reg_hash(VALUE re)
{
    st_index_t hashval = reg_hash(re);
    return LONG2FIX(hashval);
}
rxp.inspect → string

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

/ab+c/x.inspect        #=> "/ab+c/ix"
static VALUE
rb_reg_inspect(VALUE re)
{
    if (!RREGEXP(re)->ptr || !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);
}
rxp.match(str) → matchdata or nil rxp.match(str,pos) → matchdata or nil

Returns a MatchDataobject describing the match, or nilif 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

pat.match(str) {|m| ...}

instead of

if m = pat.match(str)
  ...
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;
}
rxp.named_captures → hash

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)
{
    VALUE hash = rb_hash_new();
    rb_reg_check(re);
    onig_foreach_name(RREGEXP(re)->ptr, reg_named_captures_iter, (void*)hash);
    return hash;
}
rxp.names → [name1, name2, ...]

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_ary_new();
    rb_reg_check(re);
    onig_foreach_name(RREGEXP(re)->ptr, reg_names_iter, (void*)ary);
    return ary;
}
rxp.options → fixnum

Returns the set of bits corresponding to the options used when creating this Regexp (see Regexp::newfor 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/x.options                     #=> 3
Regexp.new('cat', true).options     #=> 1
/\xa1\xa2/.options                 #=> 16

r = /cat/x
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);
}
rxp.source → str

Returns the original string of the pattern.

/ab+c/x.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_enc_str_new(RREGEXP_SRC_PTR(re),RREGEXP_SRC_LEN(re), rb_enc_get(re));
    if (OBJ_TAINTED(re)) OBJ_TAINT(str);
    return str;
}
to_json(*)

Stores class name (Regexp) with options oand source s(Regexp or String) as JSON string

# File ext/json/lib/json/add/core.rb, line 243
def to_json(*)
  as_json.to_json
end
rxp.to_s → str

Returns a string containing the regular expression and its options (using the (?opts:source)notation. This string can be fed back in to Regexp::newto 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#inspectproduces a generally more readable version of rxp.

r1 = /ab+c/x           #=> /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)
{
    int options, opt;
    const int embeddable = ONIG_OPTION_MULTILINE|ONIG_OPTION_IGNORECASE|ONIG_OPTION_EXTEND;
    long len;
    const UChar* ptr;
    VALUE str = rb_str_buf_new2("(?");
    char optbuf[5];
    rb_encoding *enc = rb_enc_get(re);

    rb_reg_check(re);

    rb_enc_copy(str, re);
    options = RREGEXP(re)->ptr->options;
    ptr = (UChar*)RREGEXP_SRC_PTR(re);
    len = RREGEXP_SRC_LEN(re);
  again:
    if (len >= 4 && ptr[0] == '(' && ptr[1] == '?') {
        int err = 1;
        ptr += 2;
        if ((len -= 2) > 0) {
            do {
                opt = char_to_option((int )*ptr);
                if (opt != 0) {
                    options |= opt;
                }
                else {
                    break;
                }
                ++ptr;
            } while (--len > 0);
        }
        if (len > 1 && *ptr == '-') {
            ++ptr;
            --len;
            do {
                opt = char_to_option((int )*ptr);
                if (opt != 0) {
                    options &= ~opt;
                }
                else {
                    break;
                }
                ++ptr;
            } while (--len > 0);
        }
        if (*ptr == ')') {
            --len;
            ++ptr;
            goto again;
        }
        if (*ptr == ':' && ptr[len-1] == ')') {
            Regexp *rp;

            ++ptr;
            len -= 2;
            err = onig_new(&rp, ptr, ptr + len, ONIG_OPTION_DEFAULT,
                           enc, OnigDefaultSyntax, NULL);
            onig_free(rp);
        }
        if (err) {
            options = RREGEXP(re)->ptr->options;
            ptr = (UChar*)RREGEXP_SRC_PTR(re);
            len = RREGEXP_SRC_LEN(re);
        }
    }

    if (*option_to_str(optbuf, options)) rb_str_buf_cat2(str, optbuf);

    if ((options & embeddable) != embeddable) {
        optbuf[0] = '-';
        option_to_str(optbuf + 1, ~options);
        rb_str_buf_cat2(str, optbuf);
    }

    rb_str_buf_cat2(str, ":");
    rb_reg_expr_str(str, (char*)ptr, len, enc, NULL);
    rb_str_buf_cat2(str, ")");
    rb_enc_copy(str, re);

    OBJ_INFECT(str, re);
    return str;
}
~ rxp → integer or nil

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

$_ = "input data"
~ /at/   #=> 7
VALUE
rb_reg_match2(VALUE re)
{
    long start;
    VALUE line = rb_lastline_get();

    if (TYPE(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);
}