Aregular expression (also called aregexp) is amatch pattern (also simply called apattern).

A common notation for a regexp uses enclosing slash characters:

/foo/

A regexp may be applied to atarget string; The part of the string (if any) that matches the pattern is called amatch, and may be saidto match:

re =/red/re.match?('redirect')# => true   # Match at beginning of target.re.match?('bored')# => true   # Match at end of target.re.match?('credit')# => true   # Match within target.re.match?('foo')# => false  # No match.

Regexp Uses¶↑

A regexp may be used:

• To extract substrings based on a given pattern:

  re =/foo/# => /foo/re.match('food')# => #<MatchData "foo">re.match('good')# => nil

  See sectionsMethod match andOperator =~.

• To determine whether a string matches a given pattern:

  re.match?('food')# => truere.match?('good')# => false

  See sectionMethod match?.

• As an argument for calls to certain methods in other classes and modules; most such methods accept an argument that may be either a string or the (much more powerful) regexp.

  SeeRegexp Methods.

Regexp Objects¶↑

A regexp object has:

• A source; seeSources.

• Several modes; seeModes.

• A timeout; seeTimeouts.

• An encoding; seeEncodings.

Creating a Regexp¶↑

A regular expression may be created with:

• A regexp literal using slash characters (seeRegexp Literals):

  # This is a very common usage./foo/# => /foo/

• A%r regexp literal (see%r: Regexp Literals):

  # Same delimiter character at beginning and end;# useful for avoiding escaping characters%r/name\/value pair/# => /name\/value pair/%r:name/value pair:# => /name\/value pair/%r|name/value pair|# => /name\/value pair/# Certain "paired" characters can be delimiters.%r[foo]# => /foo/%r{foo}# => /foo/%r(foo)# => /foo/%r<foo># => /foo/

• MethodRegexp.new.

Methodmatch¶↑

Each of the methodsRegexp#match,String#match, andSymbol#match returns aMatchData object if a match was found,nil otherwise; each also setsglobal variables:

'food'.match(/foo/)# => #<MatchData "foo">'food'.match(/bar/)# => nil

Operator=~¶↑

Each of the operatorsRegexp#=~,String#=~, andSymbol#=~ returns an integer offset if a match was found,nil otherwise; each also setsglobal variables:

/bar/=~'foo bar'# => 4'foo bar'=~/bar/# => 4/baz/=~'foo bar'# => nil

Methodmatch?¶↑

Each of the methodsRegexp#match?,String#match?, andSymbol#match? returnstrue if a match was found,false otherwise; none setsglobal variables:

'food'.match?(/foo/)# => true'food'.match?(/bar/)# => false

Global Variables¶↑

Certain regexp-oriented methods assign values to global variables:

• match: seeMethod match.

• =~: seeOperator =~.

The affected global variables are:

• $~: Returns aMatchData object, ornil.

• $&: Returns the matched part of the string, ornil.

• $`: Returns the part of the string to the left of the match, ornil.

• $': Returns the part of the string to the right of the match, ornil.

• $+: Returns the last group matched, ornil.

• $1,$2, etc.: Returns the first, second, etc., matched group, ornil. Note that$0 is quite different; it returns the name of the currently executing program.

These variables, except for$~, are shorthands for methods of$~. SeeGlobal variables equivalence atMatchData.

Examples:

# Matched string, but no matched groups.'foo bar bar baz'.match('bar')$~# => #<MatchData "bar">$&# => "bar"$`# => "foo "$'# => " bar baz"$+# => nil$1# => nil# Matched groups./s(\w{2}).*(c)/.match('haystack')$~# => #<MatchData "stac" 1:"ta" 2:"c">$&# => "stac"$`# => "hay"$'# => "k"$+# => "c"$1# => "ta"$2# => "c"$3# => nil# No match.'foo'.match('bar')$~# => nil$&# => nil$`# => nil$'# => nil$+# => nil$1# => nil

Note thatRegexp#match?,String#match?, andSymbol#match? do not set global variables.

Sources¶↑

As seen above, the simplest regexp uses a literal expression as its source:

re =/foo/# => /foo/re.match('food')# => #<MatchData "foo">re.match('good')# => nil

A rich collection of availablesubexpressions gives the regexp great power and flexibility:

• Special characters

• Source literals

• Character classes

• Shorthand character classes

• Anchors

• Alternation

• Quantifiers

• Groups and captures

• Unicode

• POSIX Bracket Expressions

• Comments

Special Characters¶↑

Regexp special characters, calledmetacharacters, have special meanings in certain contexts; depending on the context, these are sometimes metacharacters:

. ? - + * ^ \ | $ ( ) [ ] { }

To match a metacharacter literally, backslash-escape it:

# Matches one or more 'o' characters./o+/.match('foo')# => #<MatchData "oo"># Would match 'o+'./o\+/.match('foo')# => nil

To match a backslash literally, backslash-escape it:

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

MethodRegexp.escape returns an escaped string:

Regexp.escape('.?-+*^\|$()[]{}')# => "\\.\\?\\-\\+\\*\\^\\\\\\|\\$\\(\\)\\[\\]\\{\\}"

Source Literals¶↑

The source literal largely behaves like a double-quoted string; seeDouble-Quoted String Literals.

In particular, a source literal may contain interpolated expressions:

s ='foo'# => "foo"/#{s}/# => /foo//#{s.capitalize}/# => /Foo//#{2 + 2}/# => /4/

There are differences between an ordinary string literal and a source literal; seeShorthand Character Classes.

• \s in an ordinary string literal is equivalent to a space character; in a source literal, it’s shorthand for matching a whitespace character.

• In an ordinary string literal, these are (needlessly) escaped characters; in a source literal, they are shorthands for various matching characters:

  \w \W \d \D \h \H \S \R

Character Classes¶↑

Acharacter class is delimited by square brackets; it specifies that certain characters match at a given point in the target string:

# This character class will match any vowel.re =/B[aeiou]rd/re.match('Bird')# => #<MatchData "Bird">re.match('Bard')# => #<MatchData "Bard">re.match('Byrd')# => nil

A character class may contain hyphen characters to specify ranges of characters:

# These regexps have the same effect./[abcdef]/.match('foo')# => #<MatchData "f">/[a-f]/.match('foo')# => #<MatchData "f">/[a-cd-f]/.match('foo')# => #<MatchData "f">

When the first character of a character class is a caret (^), the sense of the class is inverted: it matches any characterexcept those specified.

/[^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]/

Shorthand Character Classes¶↑

Each of the following metacharacters serves as a shorthand for a character class:

• /./: Matches any character except a newline:

  /./.match('foo')# => #<MatchData "f">/./.match("\n")# => nil

• /./m: Matches any character, including a newline; seeMultiline Mode:

  /./m.match("\n")# => #<MatchData "\n">

• /\w/: Matches a word character: equivalent to[a-zA-Z0-9_]:

  /\w/.match(' foo')# => #<MatchData "f">/\w/.match(' _')# => #<MatchData "_">/\w/.match(' ')# => nil

• /\W/: Matches a non-word character: equivalent to[^a-zA-Z0-9_]:

  /\W/.match(' ')# => #<MatchData " ">/\W/.match('_')# => nil

• /\d/: Matches a digit character: equivalent to[0-9]:

  /\d/.match('THX1138')# => #<MatchData "1">/\d/.match('foo')# => nil

• /\D/: Matches a non-digit character: equivalent to[^0-9]:

  /\D/.match('123Jump!')# => #<MatchData "J">/\D/.match('123')# => nil

• /\h/: Matches a hexdigit character: equivalent to[0-9a-fA-F]:

  /\h/.match('xyz fedcba9876543210')# => #<MatchData "f">/\h/.match('xyz')# => nil

• /\H/: Matches a non-hexdigit character: equivalent to[^0-9a-fA-F]:

  /\H/.match('fedcba9876543210xyz')# => #<MatchData "x">/\H/.match('fedcba9876543210')# => nil

• /\s/: Matches a whitespace character: equivalent to/[ \t\r\n\f\v]/:

  /\s/.match('foo bar')# => #<MatchData " ">/\s/.match('foo')# => nil

• /\S/: Matches a non-whitespace character: equivalent to/[^ \t\r\n\f\v]/:

  /\S/.match(" \t\r\n\f\v foo")# => #<MatchData "f">/\S/.match(" \t\r\n\f\v")# => nil

• /\R/: Matches a linebreak, platform-independently:

  /\R/.match("\r")# => #<MatchData "\r">     # Carriage return (CR)/\R/.match("\n")# => #<MatchData "\n">     # Newline (LF)/\R/.match("\f")# => #<MatchData "\f">     # Formfeed (FF)/\R/.match("\v")# => #<MatchData "\v">     # Vertical tab (VT)/\R/.match("\r\n")# => #<MatchData "\r\n">   # CRLF/\R/.match("\u0085")# => #<MatchData "\u0085"> # Next line (NEL)/\R/.match("\u2028")# => #<MatchData "\u2028"> # Line separator (LSEP)/\R/.match("\u2029")# => #<MatchData "\u2029"> # Paragraph separator (PSEP)

Anchors¶↑

An anchor is a metasequence that matches a zero-width position between characters in the target string.

For a subexpression with no anchor, matching may begin anywhere in the target string:

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

For a subexpression with an anchor, matching must begin at the matched anchor.

Boundary Anchors¶↑

Each of these anchors matches a boundary:

• ^: Matches the beginning of a line:

  /^bar/.match("foo\nbar")# => #<MatchData "bar">/^ar/.match("foo\nbar")# => nil

• $: Matches the end of a line:

  /bar$/.match("foo\nbar")# => #<MatchData "bar">/ba$/.match("foo\nbar")# => nil

• \A: Matches the beginning of the string:

  /\Afoo/.match('foo bar')# => #<MatchData "foo">/\Afoo/.match(' foo bar')# => nil

• \Z: Matches the end of the string; if string ends with a single newline, it matches just before the ending newline:

  /foo\Z/.match('bar foo')# => #<MatchData "foo">/foo\Z/.match('foo bar')# => nil/foo\Z/.match("bar foo\n")# => #<MatchData "foo">/foo\Z/.match("bar foo\n\n")# => nil

• \z: Matches the end of the string:

  /foo\z/.match('bar foo')# => #<MatchData "foo">/foo\z/.match('foo bar')# => nil/foo\z/.match("bar foo\n")# => nil

• \b: Matches word boundary when not inside brackets; matches backspace ("0x08") when inside brackets:

  /foo\b/.match('foo bar')# => #<MatchData "foo">/foo\b/.match('foobar')# => nil

• \B: Matches non-word boundary:

  /foo\B/.match('foobar')# => #<MatchData "foo">/foo\B/.match('foo bar')# => nil

• \G: Matches first matching position:

  In methods likeString#gsub andString#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 likeRegexp#match andString#match that take an optional offset, it matches where the search begins.

  "hello, world".match(/,/,3)# => #<MatchData ",">"hello, world".match(/\G,/,3)# => nil

Lookaround Anchors¶↑

Lookahead anchors:

• (?=pat): Positive lookahead assertion: ensures that the following characters matchpat, but doesn’t include those characters in the matched substring.

• (?!pat): Negative lookahead assertion: ensures that the following charactersdo not matchpat, but doesn’t include those characters in the matched substring.

Lookbehind anchors:

• (?<=pat): Positive lookbehind assertion: ensures that the preceding characters matchpat, but doesn’t include those characters in the matched substring.

• (?<!pat): Negative lookbehind assertion: ensures that the preceding characters do not matchpat, but doesn’t include those characters in the matched substring.

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 favors the <b>bold</b>.")# => #<MatchData "bold">

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

Match-Reset Anchor¶↑

• \K: Match reset: the matched content preceding\K in the regexp is excluded from the result. For example, the following two regexps are almost equivalent:

  /ab\Kc/.match('abc')# => #<MatchData "c">/(?<=ab)c/.match('abc')# => #<MatchData "c">

  These match same string and$& equals'c', while the matched position is different.

  As are the following two regexps:

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

Alternation¶↑

The vertical bar metacharacter (|) may be used within parentheses to express alternation: two or more subexpressions any of which may match the target string.

Two alternatives:

re =/(a|b)/re.match('foo')# => nilre.match('bar')# => #<MatchData "b" 1:"b">

Four alternatives:

re =/(a|b|c|d)/re.match('shazam')# => #<MatchData "a" 1:"a">re.match('cold')# => #<MatchData "c" 1:"c">

Each alternative is a subexpression, and may be composed of other subexpressions:

re =/([a-c]|[x-z])/re.match('bar')# => #<MatchData "b" 1:"b">re.match('ooz')# => #<MatchData "z" 1:"z">

MethodRegexp.union provides a convenient way to construct a regexp with alternatives.

Quantifiers¶↑

A simple regexp matches one character:

/\w/.match('Hello')# => #<MatchData "H">

An addedquantifier specifies how many matches are required or allowed:

• * - Matches zero or more times:

  /\w*/.match('')# => #<MatchData "">/\w*/.match('x')# => #<MatchData "x">/\w*/.match('xyz')# => #<MatchData "xyz">

• + - Matches one or more times:

  /\w+/.match('')# => nil/\w+/.match('x')# => #<MatchData "x">/\w+/.match('xyz')# => #<MatchData "xyz">

• ? - Matches zero or one times:

  /\w?/.match('')# => #<MatchData "">/\w?/.match('x')# => #<MatchData "x">/\w?/.match('xyz')# => #<MatchData "x">

• {n} - Matches exactlyn times:

  /\w{2}/.match('')# => nil/\w{2}/.match('x')# => nil/\w{2}/.match('xyz')# => #<MatchData "xy">

• {min,} - Matchesmin or more times:

  /\w{2,}/.match('')# => nil/\w{2,}/.match('x')# => nil/\w{2,}/.match('xy')# => #<MatchData "xy">/\w{2,}/.match('xyz')# => #<MatchData "xyz">

• {,max} - Matchesmax or fewer times:

  /\w{,2}/.match('')# => #<MatchData "">/\w{,2}/.match('x')# => #<MatchData "x">/\w{,2}/.match('xyz')# => #<MatchData "xy">

• {min,max} - Matches at leastmin times and at mostmax times:

  /\w{1,2}/.match('')# => nil/\w{1,2}/.match('x')# => #<MatchData "x">/\w{1,2}/.match('xyz')# => #<MatchData "xy">

Greedy, Lazy, or Possessive Matching¶↑

Quantifier matching may be greedy, lazy, or possessive:

• Ingreedy matching, as many occurrences as possible are matched while still allowing the overall match to succeed. Greedy quantifiers:*,+,?,{min, max} and its variants.

• Inlazy matching, the minimum number of occurrences are matched. Lazy quantifiers:*?,+?,??,{min, max}? and its variants.

• Inpossessive matching, once a match is found, there is no backtracking; that match is retained, even if it jeopardises the overall match. Possessive quantifiers:*+,++,?+. Note that{min, max} and its variants donot support possessive matching.

More:

• About greedy and lazy matching, seeChoosing Minimal or Maximal Repetition.

• About possessive matching, seeEliminate Needless Backtracking.

Groups and Captures¶↑

A simple regexp has (at most) one match:

re =/\d\d\d\d-\d\d-\d\d/re.match('1943-02-04')# => #<MatchData "1943-02-04">re.match('1943-02-04').size# => 1re.match('foo')# => nil

Adding one or more pairs of parentheses,(subexpression), definesgroups, which may result in multiple matched substrings, calledcaptures:

re =/(\d\d\d\d)-(\d\d)-(\d\d)/re.match('1943-02-04')# => #<MatchData "1943-02-04" 1:"1943" 2:"02" 3:"04">re.match('1943-02-04').size# => 4

The first capture is the entire matched string; the other captures are the matched substrings from the groups.

A group may have aquantifier:

re =/July 4(th)?/re.match('July 4')# => #<MatchData "July 4" 1:nil>re.match('July 4th')# => #<MatchData "July 4th" 1:"th">re =/(foo)*/re.match('')# => #<MatchData "" 1:nil>re.match('foo')# => #<MatchData "foo" 1:"foo">re.match('foofoo')# => #<MatchData "foofoo" 1:"foo">re =/(foo)+/re.match('')# => nilre.match('foo')# => #<MatchData "foo" 1:"foo">re.match('foofoo')# => #<MatchData "foofoo" 1:"foo">

The returned MatchData object gives access to the matched substrings:

re =/(\d\d\d\d)-(\d\d)-(\d\d)/md =re.match('1943-02-04')# => #<MatchData "1943-02-04" 1:"1943" 2:"02" 3:"04">md[0]# => "1943-02-04"md[1]# => "1943"md[2]# => "02"md[3]# => "04"

Non-Capturing Groups¶↑

A group may be made non-capturing; it is still a group (and, for example, can have a quantifier), but its matching substring is not included among the captures.

A non-capturing group begins with?: (inside the parentheses):

# Don't capture the year.re =/(?:\d\d\d\d)-(\d\d)-(\d\d)/md =re.match('1943-02-04')# => #<MatchData "1943-02-04" 1:"02" 2:"04">

Backreferences¶↑

A group match may also be referenced within the regexp itself; such a reference is called abackreference:

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

This table shows how each subexpression in the regexp above matches a substring in the target string:

| Subexpression in Regexp   | Matching Substring in Target String ||---------------------------|-------------------------------------||       First '[csh]'       |            Character 'c'            ||          '(..)'           |        First substring 'at'         ||      First space ' '      |      First space character ' '      ||       Second '[csh]'      |            Character 's'            || '\1' (backreference 'at') |        Second substring 'at'        ||           ' in'           |            Substring ' in'          |

A regexp may contain any number of groups:

• For a large number of groups:

  • The ordinary\n notation applies only forn in range (1..9).

  • TheMatchData[n] notation applies for any non-negativen.

• \0 is a special backreference, referring to the entire matched string; it may not be used within the regexp itself, but may be used outside it (for example, in a substitution method call):

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

Named Captures¶↑

As seen above, a capture can be referred to by its number. A capture can also have a name, prefixed as?<name> or?'name', and the name (symbolized) may be used as an index inMatchData[]:

md =/\$(?<dollars>\d+)\.(?'cents'\d+)/.match("$3.67")# => #<MatchData "$3.67" dollars:"3" cents:"67">md[:dollars]# => "3"md[:cents]# => "67"# The capture numbers are still valid.md[2]# => "67"

When a regexp contains a named capture, there are no unnamed captures:

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

A named group may be backreferenced as\k<name>:

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

When (and only when) a regexp contains named capture groups and appears before the=~ operator, the captured substrings are assigned to local variables with corresponding names:

/\$(?<dollars>\d+)\.(?<cents>\d+)/=~'$3.67'dollars# => "3"cents# => "67"

MethodRegexp#named_captures returns a hash of the capture names and substrings; methodRegexp#names returns an array of the capture names.

Atomic Grouping¶↑

A group may be madeatomic with(?>subexpression).

This causes the subexpression to be matched independently of the rest of the expression, so that the matched substring becomes fixed for the remainder of the match, unless the entire subexpression must be abandoned and subsequently revisited.

In this waysubexpression is treated as a non-divisible whole. Atomic grouping is typically used to optimise patterns to prevent needless backtracking .

Example (without atomic grouping):

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

Analysis:

1. The leading subexpression" in the pattern matches the first character" in the target string.

2. The next subexpression.* matches the next substringQuote" (including the trailing double-quote).

3. Now there is nothing left in the target string to match the trailing subexpression" in the pattern; this would cause the overall match to fail.

4. The matched substring is backtracked by one position:Quote.

5. The final subexpression" now matches the final substring", and the overall match succeeds.

If subexpression.* is grouped atomically, the backtracking is disabled, and the overall match fails:

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

Atomic grouping can affect performance; seeAtomic Group.

Subexpression Calls¶↑

As seen above, a backreference number (\n) or name (\k<name>) gives access to a capturedsubstring; the corresponding regexpsubexpression may also be accessed, via the number (\gn) or name (\g<name>):

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

The pattern:

1. Matches at the beginning of the string, i.e. before the first character.

2. Enters a named groupparen.

3. Matches the first character in the string,'('.

4. Calls theparen group again, i.e. recurses back to the second step.

5. Re-enters theparen group.

6. Matches the second character in the string,'('.

7. Attempts to callparen a third time, but fails because doing so would prevent an overall successful match.

8. Matches the third character in the string,')'; marks the end of the second recursive call

9. Matches the fourth character in the string,')'.

10. Matches the end of the string.

SeeSubexpression calls.

Conditionals¶↑

The conditional construct takes the form(?(cond)yes|no), where:

• cond may be a capture number or name.

• The match to be applied isyes ifcond is captured; otherwise the match to be applied isno.

• If not needed,|no may be omitted.

Examples:

re =/\A(foo)?(?(1)(T)|(F))\z/re.match('fooT')# => #<MatchData "fooT" 1:"foo" 2:"T" 3:nil>re.match('F')# => #<MatchData "F" 1:nil 2:nil 3:"F">re.match('fooF')# => nilre.match('T')# => nilre =/\A(?<xyzzy>foo)?(?(<xyzzy>)(T)|(F))\z/re.match('fooT')# => #<MatchData "fooT" xyzzy:"foo">re.match('F')# => #<MatchData "F" xyzzy:nil>re.match('fooF')# => nilre.match('T')# => nil

Absence Operator¶↑

The absence operator is a special group that matches anything which doesnot match the contained subexpressions.

/(?~real)/.match('surrealist')# => #<MatchData "surrea">/(?~real)ist/.match('surrealist')# => #<MatchData "ealist">/sur(?~real)ist/.match('surrealist')# => nil

Unicode¶↑

Unicode Properties¶↑

The/\p{property_name}/ construct (with lowercasep) matches characters using a Unicode property name, much like a character class; propertyAlpha specifies alphabetic characters:

/\p{Alpha}/.match('a')# => #<MatchData "a">/\p{Alpha}/.match('1')# => nil

A property can be inverted by prefixing the name with a caret character (^):

/\p{^Alpha}/.match('1')# => #<MatchData "1">/\p{^Alpha}/.match('a')# => nil

Or by using\P (uppercaseP):

/\P{Alpha}/.match('1')# => #<MatchData "1">/\P{Alpha}/.match('a')# => nil

SeeUnicode Properties for regexps based on the numerous properties.

Some commonly-used properties correspond to POSIX bracket expressions:

• /\p{Alnum}/: Alphabetic and numeric character

• /\p{Alpha}/: Alphabetic character

• /\p{Blank}/: Space or tab

• /\p{Cntrl}/: Control character

• /\p{Digit}/: Digit 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)

These are also commonly used:

• /\p{Emoji}/: Unicode emoji.

• /\p{Graph}/: Characters excluding/\p{Cntrl}/ and/\p{Space}/. Note that invisible characters under the Unicode“Format” category are included.

• /\p{Word}/: A member in one of these Unicode character categories (see below) or having one of these Unicode properties:

  • Unicode categories:

    • Mark (M).

    • Decimal Number (Nd)

    • Connector Punctuation (Pc).

  • Unicode properties:

    • Alpha

    • Join_Control

• /\p{ASCII}/: A character in the ASCII character set.

• /\p{Any}/: Any Unicode character (including unassigned characters).

• /\p{Assigned}/: An assigned character.

Unicode Character Categories¶↑

A Unicode character category name:

• May be either its full name or its abbreviated name.

• Is case-insensitive.

• Treats a space, a hyphen, and an underscore as equivalent.

Examples:

/\p{lu}/# => /\p{lu}//\p{LU}/# => /\p{LU}//\p{Uppercase Letter}/# => /\p{Uppercase Letter}//\p{Uppercase_Letter}/# => /\p{Uppercase_Letter}//\p{UPPERCASE-LETTER}/# => /\p{UPPERCASE-LETTER}/

Below are the Unicode character category abbreviations and names. Enumerations of characters in each category are at the links.

Letters:

• L,Letter:LC,Lm, orLo.

• LC,Cased_Letter:Ll,Lt, orLu.

• Lu, Lowercase_Letter.

• Lu, Modifier_Letter.

• Lu, Other_Letter.

• Lu, Titlecase_Letter.

• Lu, Uppercase_Letter.

Marks:

• M,Mark:Mc,Me, orMn.

• Mc, Spacing_Mark.

• Me, Enclosing_Mark.

• Mn, Nonapacing_Mark.

Numbers:

• N,Number:Nd,Nl, orNo.

• Nd, Decimal_Number.

• Nl, Letter_Number.

• No, Other_Number.

Punctuation:

• P,Punctuation:Pc,Pd,Pe,Pf,Pi,Po, orPs.

• Pc, Connector_Punctuation.

• Pd, Dash_Punctuation.

• Pe, Close_Punctuation.

• Pf, Final_Punctuation.

• Pi, Initial_Punctuation.

• Po, Other_Punctuation.

• Ps, Open_Punctuation.

• S,Symbol:Sc,Sk,Sm, orSo.

• Sc, Currency_Symbol.

• Sk, Modifier_Symbol.

• Sm, Math_Symbol.

• So, Other_Symbol.

• Z,Separator:Zl,Zp, orZs.

• Zl, Line_Separator.

• Zp, Paragraph_Separator.

• Zs, Space_Separator.

• C,Other:Cc,Cf,Cn,Co, orCs.

• Cc, Control.

• Cf, Format.

• Cn, Unassigned.

• Co, Private_Use.

• Cs, Surrogate.

Unicode Scripts and Blocks¶↑

Among the Unicode properties are:

• Unicode scripts; seesupported scripts.

• Unicode blocks; seesupported blocks.

POSIX Bracket Expressions¶↑

A POSIXbracket expression is also similar to a character class. These expressions provide a portable alternative to the above, with the added benefit of encompassing non-ASCII characters:

• /\d/ matches only ASCII decimal digits0 through9.

• /[[:digit:]]/ matches any character in the UnicodeDecimal Number (Nd) category; see below.

The POSIX bracket expressions:

• /[[:digit:]]/: Matches aUnicode digit:

  /[[:digit:]]/.match('9')# => #<MatchData "9">/[[:digit:]]/.match("\u1fbf9")# => #<MatchData "9">

• /[[:xdigit:]]/: Matches a digit allowed in a hexadecimal number; equivalent to[0-9a-fA-F].

• /[[:upper:]]/: Matches aUnicode uppercase letter:

  /[[:upper:]]/.match('A')# => #<MatchData "A">/[[:upper:]]/.match("\u00c6")# => #<MatchData "Æ">

• /[[:lower:]]/: Matches aUnicode lowercase letter:

  /[[:lower:]]/.match('a')# => #<MatchData "a">/[[:lower:]]/.match("\u01fd")# => #<MatchData "ǽ">

• /[[:alpha:]]/: Matches/[[:upper:]]/ or/[[:lower:]]/.

• /[[:alnum:]]/: Matches/[[:alpha:]]/ or/[[:digit:]]/.

• /[[:space:]]/: MatchesUnicode space character:

  /[[:space:]]/.match(' ')# => #<MatchData " ">/[[:space:]]/.match("\u2005")# => #<MatchData " ">

• /[[:blank:]]/: Matches/[[:space:]]/ or tab character:

  /[[:blank:]]/.match(' ')# => #<MatchData " ">/[[:blank:]]/.match("\u2005")# => #<MatchData " ">/[[:blank:]]/.match("\t")# => #<MatchData "\t">

• /[[:cntrl:]]/: MatchesUnicode control character:

  /[[:cntrl:]]/.match("\u0000")# => #<MatchData "\u0000">/[[:cntrl:]]/.match("\u009f")# => #<MatchData "\u009F">

• /[[:graph:]]/: Matches any character except/[[:space:]]/ or/[[:cntrl:]]/.

• /[[:print:]]/: Matches/[[:graph:]]/ or space character.

• /[[:punct:]]/: Matches any (Unicode punctuation character}[www.compart.com/en/unicode/category/Po]:

Ruby also supports these (non-POSIX) bracket expressions:

• /[[:ascii:]]/: Matches a character in the ASCII character set.

• /[[:word:]]/: Matches a character in one of these Unicode character categories or having one of these Unicode properties:

  • Unicode categories:

    • Mark (M).

    • Decimal Number (Nd)

    • Connector Punctuation (Pc).

  • Unicode properties:

    • Alpha

    • Join_Control

Comments¶↑

A comment may be included in a regexp pattern using the(?#comment) construct, wherecomment is a substring that is to be ignored. arbitrary text ignored by the regexp engine:

/foo(?#Ignore me)bar/.match('foobar')# => #<MatchData "foobar">

The comment may not include an unescaped terminator character.

See alsoExtended Mode.

Modes¶↑

Each of these modifiers sets a mode for the regexp:

• i:/pattern/i setsCase-Insensitive Mode.

• m:/pattern/m setsMultiline Mode.

• x:/pattern/x setsExtended Mode.

• o:/pattern/o setsInterpolation Mode.

Any, all, or none of these may be applied.

Modifiersi,m, andx may be applied to subexpressions:

• (?modifier) turns the mode “on” for ensuing subexpressions

• (?-modifier) turns the mode “off” for ensuing subexpressions

• (?modifier:subexp) turns the mode “on” forsubexp within the group

• (?-modifier:subexp) turns the mode “off” forsubexp within the group

Example:

re =/(?i)te(?-i)st/re.match('test')# => #<MatchData "test">re.match('TEst')# => #<MatchData "TEst">re.match('TEST')# => nilre.match('teST')# => nilre =/t(?i:e)st/re.match('test')# => #<MatchData "test">re.match('tEst')# => #<MatchData "tEst">re.match('tEST')# => nil

MethodRegexp#options returns an integer whose value showing the settings for case-insensitivity mode, multiline mode, and extended mode.

Case-Insensitive Mode¶↑

By default, a regexp is case-sensitive:

/foo/.match('FOO')# => nil

Modifieri enables case-insensitive mode:

/foo/i.match('FOO')# => #<MatchData "FOO">

MethodRegexp#casefold? returns whether the mode is case-insensitive.

Multiline Mode¶↑

The multiline-mode in Ruby is what is commonly called a “dot-all mode”:

• Without them modifier, the subexpression. does not match newlines:

  /a.c/.match("a\nc")# => nil

• With the modifier, it does match:

  /a.c/m.match("a\nc")# => #<MatchData "a\nc">

Unlike other languages, the modifierm does not affect the anchors^ and$. These anchors always match at line-boundaries in Ruby.

Extended Mode¶↑

Modifierx enables extended mode, which means that:

• Literal white space in the pattern is to be ignored.

• Character# marks the remainder of its containing line as a comment, which is also to be ignored for matching purposes.

In extended mode, whitespace and comments may be used to form a self-documented regexp.

Regexp not in extended mode (matches some Roman numerals):

pattern ='^M{0,3}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})$'re =/#{pattern}/re.match('MCMXLIII')# => #<MatchData "MCMXLIII" 1:"CM" 2:"XL" 3:"III">

Regexp in extended mode:

pattern =<<-EOT  ^                   # beginning of string  M{0,3}              # thousands - 0 to 3 Ms  (CM|CD|D?C{0,3})    # hundreds - 900 (CM), 400 (CD), 0-300 (0 to 3 Cs),                      #            or 500-800 (D, followed by 0 to 3 Cs)  (XC|XL|L?X{0,3})    # tens - 90 (XC), 40 (XL), 0-30 (0 to 3 Xs),                      #        or 50-80 (L, followed by 0 to 3 Xs)  (IX|IV|V?I{0,3})    # ones - 9 (IX), 4 (IV), 0-3 (0 to 3 Is),                      #        or 5-8 (V, followed by 0 to 3 Is)  $                   # end of stringEOTre =/#{pattern}/xre.match('MCMXLIII')# => #<MatchData "MCMXLIII" 1:"CM" 2:"XL" 3:"III">

Interpolation Mode¶↑

Modifiero means that the first time a literal regexp with interpolations is encountered, the generatedRegexp object is saved and used for all future evaluations of that literal regexp. Without modifiero, the generatedRegexp is not saved, so each evaluation of the literal regexp generates a newRegexp object.

Without modifiero:

defletters;sleep5;/[A-Z][a-z]/;endwords =%w[abc def xyz]start =Time.nowwords.each {|word|word.match(/\A[#{letters}]+\z/) }Time.now-start# => 15.0174892

With modifiero:

start =Time.nowwords.each {|word|word.match(/\A[#{letters}]+\z/o) }Time.now-start# => 5.0010866

Note that if the literal regexp does not have interpolations, theo behavior is the default.

Encodings¶↑

By default, a regexp with only US-ASCII characters has US-ASCII encoding:

re =/foo/re.source.encoding# => #<Encoding:US-ASCII>re.encoding# => #<Encoding:US-ASCII>

A regular expression containing non-US-ASCII characters is assumed to use the source encoding. This can be overridden with one of the following modifiers.

• /pat/n: US-ASCII if only containing US-ASCII characters, otherwise ASCII-8BIT:

  /foo/n.encoding# => #<Encoding:US-ASCII>/foo\xff/n.encoding# => #<Encoding:ASCII-8BIT>/foo\x7f/n.encoding# => #<Encoding:US-ASCII>

• /pat/u: UTF-8

  /foo/u.encoding# => #<Encoding:UTF-8>

• /pat/e: EUC-JP

  /foo/e.encoding# => #<Encoding:EUC-JP>

• /pat/s: Windows-31J

  /foo/s.encoding# => #<Encoding:Windows-31J>

A regexp can be matched against a target string when either:

• They have the same encoding.

• The regexp’s encoding is a fixed encoding and the string contains only ASCII characters. MethodRegexp#fixed_encoding? returns whether the regexp has afixed encoding.

If a match between incompatible encodings is attempted anEncoding::CompatibilityError exception is raised.

Example:

re =eval("# encoding: ISO-8859-1\n/foo\\xff?/")re.encoding# => #<Encoding:ISO-8859-1>re=~"foo".encode("UTF-8")# => 0re=~"foo\u0100"# Raises Encoding::CompatibilityError

The encoding may be explicitly fixed by includingRegexp::FIXEDENCODING in the second argument forRegexp.new:

# Regexp with encoding ISO-8859-1.re =Regexp.new("a".force_encoding('iso-8859-1'),Regexp::FIXEDENCODING)re.encoding# => #<Encoding:ISO-8859-1># Target string with encoding UTF-8.s ="a\u3042"s.encoding# => #<Encoding:UTF-8>re.match(s)# Raises Encoding::CompatibilityError.

Timeouts¶↑

When either a regexp source or a target string comes from untrusted input, malicious values could become a denial-of-service attack; to prevent such an attack, it is wise to set a timeout.

Regexp has two timeout values:

• A class default timeout, used for a regexp whose instance timeout isnil; this default is initiallynil, and may be set by methodRegexp.timeout=:

  Regexp.timeout# => nilRegexp.timeout =3.0Regexp.timeout# => 3.0

• An instance timeout, which defaults tonil and may be set inRegexp.new:

  re =Regexp.new('foo',timeout:5.0)re.timeout# => 5.0

When regexp.timeout isnil, the timeout “falls through” toRegexp.timeout; when regexp.timeout is non-nil, that value controls timing out:

| regexp.timeout Value | Regexp.timeout Value |            Result           ||----------------------|----------------------|-----------------------------||         nil          |          nil         |       Never times out.      ||         nil          |         Float        | Times out in Float seconds. ||        Float         |          Any         | Times out in Float seconds. |

Optimization¶↑

For certain values of the pattern and target string, matching time can grow polynomially or exponentially in relation to the input size; the potential vulnerability arising from this is theregular expression denial-of-service (ReDoS) attack.

Regexp matching can apply an optimization to prevent ReDoS attacks. When the optimization is applied, matching time increases linearly (not polynomially or exponentially) in relation to the input size, and a ReDoS attack is not possible.

This optimization is applied if the pattern meets these criteria:

• No backreferences.

• No subexpression calls.

• No nested lookaround anchors or atomic groups.

• No nested quantifiers with counting (i.e. no nested{n},{min,},{,max}, or{min,max} style quantifiers)

You can use methodRegexp.linear_time? to determine whether a pattern meets these criteria:

Regexp.linear_time?(/a*/)# => trueRegexp.linear_time?('a*')# => trueRegexp.linear_time?(/(a*)\1/)# => false

However, an untrusted source may not be safe even if the method returnstrue, because the optimization uses memoization (which may invoke large memory consumption).

References¶↑

Read:

• Mastering Regular Expressions by Jeffrey E.F. Friedl.

• Regular Expressions Cookbook by Jan Goyvaerts & Steven Levithan.

Explore, test:

• Rubular: interactive online editor.