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.

# '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 s followed 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>\\.

/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 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:

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_])

  • /\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 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.,


Ruby also supports the following non-POSIX character classes:

  • /[[:word:]]/ - A character in one of the following Unicode

general categories _Letter_, _Mark_, _Number_,
  • /[[:ascii:]]/ - A character in the ASCII character set

    /[[:digit:]]/.match("\u06F2")    #=> #<MatchData "\u{06F2}">
    /[[:upper:]][[:lower:]]/.match("Hello") #=> #<MatchData "He">
    /[[:xdigit:]][[:xdigit:]]/.match("A6")  #=> #<MatchData "A6">


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 <i>m</i> 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. A greedy metacharacter can be made lazy by following it with ?.

# Both patterns below match the string. The fist 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.


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.

    => #<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.

    #=> #<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"


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:
# '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
    #=> #<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'.
    #=> #<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 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

  1. Enters a named capture group called paren

  2. Matches a literal (, the first character in the string

  3. Calls the paren group again, i.e. recurses back to the

second step
  1. Re-enters the paren group

  2. Matches a literal (, the second character in the

  1. Try to call paren a third time, but fail because

doing so would prevent an overall successful match
  1. Match a literal ), the third character in the string.

Marks the end of the second recursive call
  1. Match a literal ), the fourth character in the string

  2. Match the end of the string


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 <i>Letter</i>, <i>Mark</i>, <i>Number</i>,
  • /\p{ASCII}/ - A character in the ASCII character set

  • /\p{Any}/ - Any Unicode character (including unassigned

  • /\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 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 point where last match finished

  • \b - Matches word boundaries when outside brackets; backspace (0x08) inside brackets

  • \B - Matches non-word boundaries

  • (?=pat) - Positive lookahead assertion:

ensures that the following characters match <i>pat</i>, but doesn't
include those characters in the matched text
  • (?!pat) - Negative lookahead assertion:

ensures that the following characters do not match <i>pat</i>, but
doesn't include those characters in the matched text
  • (?<=pat) - Positive lookbehind

assertion: ensures that the preceding characters match <i>pat</i>, but
doesn't include those characters in the matched text
  • (?<!pat) - Negative lookbehind

assertion: ensures that the preceding characters do not match
<i>pat</i>, 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.
  # 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">


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

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 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
float_pat.match('3.14') #=> #<MatchData "3.14" 1:".14">

Note: To match whitespace in an x pattern use an escape such as \s or \p{Space}.

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


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"
   #=> Encoding::CompatibilityError: incompatible encoding regexp match
        (ISO-8859-1 regexp with UTF-8 string)


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">







Show files where this class is defined (2 files)
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February 10, 2009
3 thanks

Cheat Sheet

I have written a short introduction and a colorful cheat sheet for Perl Compatible Regular Expressions (PCRE) as used by Ruby’s Regexp class:


December 3, 2009 - (v1_8_6_287 - v1_8_7_72)
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Testing Regular Expressions?

Writing regular expressions is never easy. You may use http://rubular.com to test and optimize your regular expressions.

March 8, 2010
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Check out this simple example:

"Hello Ruby friend".sub(/^(.*)e/,  'X')  # => "Xnd"
"Hello Ruby friend".sub(/^(.*?)e/, 'X')  # => "Xllo Ruby friend"

The question mark turns the dotstar into non-eager mode which means it will halt on the first subsequent “e” rather than the last one. This comes in handy e.g. for Cucumber step definitions.

Okay, but not really nice:

/^I am using rvm "([^\"]*)" with gemset "(.*)"$/

Much more readable and consistent equivalent to the above:

/^I am using rvm "(.*?)" with gemset "(.*?)"$/