Raised when the arguments are wrong and there isn’t a more specific Exception class.
Ex: passing the wrong number of arguments
[1, 2, 3].first(4, 5)
raises the exception:
ArgumentError: wrong number of arguments (given 2, expected 1)
Ex: passing an argument that is not acceptable:
[1, 2, 3].first(-4)
raises the exception:
ArgumentError: negative array size
Raised when the given index is invalid.
a = [:foo, :bar] a.fetch(0) #=> :foo a[4] #=> nil a.fetch(4) #=> IndexError: index 4 outside of array bounds: -2...2
Raised when the specified key is not found. It is a subclass of IndexError.
h = {"foo" => :bar} h.fetch("foo") #=> :bar h.fetch("baz") #=> KeyError: key not found: "baz"
Raised when a given numerical value is out of range.
[1, 2, 3].drop(1 << 100)
raises the exception:
RangeError: bignum too big to convert into `long'
ScriptError is the superclass for errors raised when a script can not be executed because of a LoadError, NotImplementedError or a SyntaxError. Note these type of ScriptErrors are not StandardError and will not be rescued unless it is specified explicitly (or its ancestor Exception).
Raised when encountering Ruby code with an invalid syntax.
eval("1+1=2")
raises the exception:
SyntaxError: (eval):1: syntax error, unexpected '=', expecting $end
Raised when a file required (a Ruby script, extension library, …) fails to load.
require 'this/file/does/not/exist'
raises the exception:
LoadError: no such file to load -- this/file/does/not/exist
Raised when a feature is not implemented on the current platform. For example, methods depending on the fsync or fork system calls may raise this exception if the underlying operating system or Ruby runtime does not support them.
Note that if fork raises a NotImplementedError, then respond_to?(:fork) returns false.
Raised when a given name is invalid or undefined.
puts foo
raises the exception:
NameError: undefined local variable or method `foo' for main:Object
Since constant names must start with a capital:
Integer.const_set :answer, 42
raises the exception:
NameError: wrong constant name answer
Raised when a method is called on a receiver which doesn’t have it defined and also fails to respond with method_missing.
"hello".to_ary
raises the exception:
NoMethodError: undefined method `to_ary' for "hello":String
A generic error class raised when an invalid operation is attempted. Kernel#raise will raise a RuntimeError if no Exception class is specified.
raise "ouch"
raises the exception:
RuntimeError: ouch
Raised when there is an attempt to modify a frozen object.
[1, 2, 3].freeze << 4
raises the exception:
FrozenError: can't modify frozen Array
No longer used by internal code.
EncodingError is the base class for encoding errors.
An OpenStruct is a data structure, similar to a Hash, that allows the definition of arbitrary attributes with their accompanying values. This is accomplished by using Ruby’s metaprogramming to define methods on the class itself.
require "ostruct" person = OpenStruct.new person.name = "John Smith" person.age = 70 person.name # => "John Smith" person.age # => 70 person.address # => nil
An OpenStruct employs a Hash internally to store the attributes and values and can even be initialized with one:
australia = OpenStruct.new(:country => "Australia", :capital => "Canberra") # => #<OpenStruct country="Australia", capital="Canberra">
Hash keys with spaces or characters that could normally not be used for method calls (e.g. ()[]*) will not be immediately available on the OpenStruct object as a method for retrieval or assignment, but can still be reached through the Object#send method or using [].
measurements = OpenStruct.new("length (in inches)" => 24) measurements[:"length (in inches)"] # => 24 measurements.send("length (in inches)") # => 24 message = OpenStruct.new(:queued? => true) message.queued? # => true message.send("queued?=", false) message.queued? # => false
Removing the presence of an attribute requires the execution of the delete_field method as setting the property value to nil will not remove the attribute.
first_pet = OpenStruct.new(:name => "Rowdy", :owner => "John Smith") second_pet = OpenStruct.new(:name => "Rowdy") first_pet.owner = nil first_pet # => #<OpenStruct name="Rowdy", owner=nil> first_pet == second_pet # => false first_pet.delete_field(:owner) first_pet # => #<OpenStruct name="Rowdy"> first_pet == second_pet # => true
Ractor compatibility: A frozen OpenStruct with shareable values is itself shareable.
An OpenStruct utilizes Ruby’s method lookup structure to find and define the necessary methods for properties. This is accomplished through the methods method_missing and define_singleton_method.
This should be a consideration if there is a concern about the performance of the objects that are created, as there is much more overhead in the setting of these properties compared to using a Hash or a Struct. Creating an open struct from a small Hash and accessing a few of the entries can be 200 times slower than accessing the hash directly.
This is a potential security issue; building OpenStruct from untrusted user data (e.g. JSON web request) may be susceptible to a “symbol denial of service” attack since the keys create methods and names of methods are never garbage collected.
This may also be the source of incompatibilities between Ruby versions:
o = OpenStruct.new o.then # => nil in Ruby < 2.6, enumerator for Ruby >= 2.6
Builtin methods may be overwritten this way, which may be a source of bugs or security issues:
o = OpenStruct.new o.methods # => [:to_h, :marshal_load, :marshal_dump, :each_pair, ... o.methods = [:foo, :bar] o.methods # => [:foo, :bar]
To help remedy clashes, OpenStruct uses only protected/private methods ending with ! and defines aliases for builtin public methods by adding a !:
o = OpenStruct.new(make: 'Bentley', class: :luxury) o.class # => :luxury o.class! # => OpenStruct
It is recommended (but not enforced) to not use fields ending in !; Note that a subclass’ methods may not be overwritten, nor can OpenStruct’s own methods ending with !.
For all these reasons, consider not using OpenStruct at all.
Regular expressions (regexps) are patterns which describe the contents of a string. They’re used for testing whether a string contains a given pattern, or extracting the portions that match. They are created with the /pat/ and %r{pat} literals or the Regexp.new constructor.
A regexp is usually delimited with forward slashes (/). For example:
/hay/ =~ 'haystack' #=> 0 /y/.match('haystack') #=> #<MatchData "y">
If a string contains the pattern it is said to match. A literal string matches itself.
Here ‘haystack’ does not contain the pattern ‘needle’, so it doesn’t match:
/needle/.match('haystack') #=> nil
Here ‘haystack’ contains the pattern ‘hay’, so it matches:
/hay/.match('haystack') #=> #<MatchData "hay">
Specifically, /st/ requires that the string contains the letter s followed by the letter t, so it matches haystack, also.
Note that any Regexp matching will raise a RuntimeError if timeout is set and exceeded. See “Timeout” section in detail.
A regexp may contain interpolated strings; trivially:
foo = 'bar' /#{foo}/ # => /bar/
=~ and Regexp#match Pattern matching may be achieved by using =~ operator or Regexp#match method.
=~ Operator =~ is Ruby’s basic pattern-matching operator. When one operand is a regular expression and the other is a string then the regular expression is used as a pattern to match against the string. (This operator is equivalently defined by Regexp and String so the order of String and Regexp do not matter. Other classes may have different implementations of =~.) If a match is found, the operator returns index of first match in string, otherwise it returns nil.
/hay/ =~ 'haystack' #=> 0 'haystack' =~ /hay/ #=> 0 /a/ =~ 'haystack' #=> 1 /u/ =~ 'haystack' #=> nil
Using =~ operator with a String and Regexp the $~ global variable is set after a successful match. $~ holds a MatchData object. Regexp.last_match is equivalent to $~.
Regexp#match Method The match method returns a MatchData object:
/st/.match('haystack') #=> #<MatchData "st">
The following are metacharacters (, ), [, ], {, }, ., ?, +, *. They have a specific meaning when appearing in a pattern. To match them literally they must be backslash-escaped. To match a backslash literally, backslash-escape it: \\.
/1 \+ 2 = 3\?/.match('Does 1 + 2 = 3?') #=> #<MatchData "1 + 2 = 3?"> /a\\\\b/.match('a\\\\b') #=> #<MatchData "a\\b">
Patterns behave like double-quoted strings and can contain the same backslash escapes (the meaning of \s is different, however, see below).
/\s\u{6771 4eac 90fd}/.match("Go to 東京都") #=> #<MatchData " 東京都">
Arbitrary Ruby expressions can be embedded into patterns with the #{...} construct.
place = "東京都" /#{place}/.match("Go to 東京都") #=> #<MatchData "東京都">
A character class is delimited with square brackets ([, ]) and lists characters that may appear at that point in the match. /[ab]/ means a or b, as opposed to /ab/ which means a followed by b.
/W[aeiou]rd/.match("Word") #=> #<MatchData "Word">
Within a character class the hyphen (-) is a metacharacter denoting an inclusive range of characters. [abcd] is equivalent to [a-d]. A range can be followed by another range, so [abcdwxyz] is equivalent to [a-dw-z]. The order in which ranges or individual characters appear inside a character class is irrelevant.
/[0-9a-f]/.match('9f') #=> #<MatchData "9"> /[9f]/.match('9f') #=> #<MatchData "9">
If the first character of a character class is a caret (^) the class is inverted: it matches any character except those named.
/[^a-eg-z]/.match('f') #=> #<MatchData "f">
A character class may contain another character class. By itself this isn’t useful because [a-z[0-9]] describes the same set as [a-z0-9]. However, character classes also support the && operator which performs set intersection on its arguments. The two can be combined as follows:
/[a-w&&[^c-g]z]/ # ([a-w] AND ([^c-g] OR z))
This is equivalent to:
/[abh-w]/
The following metacharacters also behave like character classes:
/./ - Any character except a newline.
/./m - Any character (the m modifier enables multiline mode)
/\w/ - A word character ([a-zA-Z0-9_])
/\W/ - A non-word character ([^a-zA-Z0-9_]). Please take a look at Bug #4044 if using /\W/ with the /i modifier.
/\d/ - A digit character ([0-9])
/\D/ - A non-digit character ([^0-9])
/\h/ - A hexdigit character ([0-9a-fA-F])
/\H/ - A non-hexdigit character ([^0-9a-fA-F])
/\s/ - A whitespace character: /[ \t\r\n\f\v]/
/\S/ - A non-whitespace character: /[^ \t\r\n\f\v]/
/\R/ - A linebreak: \n, \v, \f, \r \u0085 (NEXT LINE), \u2028 (LINE SEPARATOR), \u2029 (PARAGRAPH SEPARATOR) or \r\n.
POSIX bracket expressions are also similar to character classes. They provide a portable alternative to the above, with the added benefit that they encompass non-ASCII characters. For instance, /\d/ matches only the ASCII decimal digits (0-9); whereas /[[:digit:]]/ matches any character in the Unicode Nd category.
/[[:alnum:]]/ - Alphabetic and numeric character
/[[:alpha:]]/ - Alphabetic character
/[[:blank:]]/ - Space or tab
/[[:cntrl:]]/ - Control character
/[[:digit:]]/ - Digit
/[[:graph:]]/ - Non-blank character (excludes spaces, control characters, and similar)
/[[:lower:]]/ - Lowercase alphabetical character
/[[:print:]]/ - Like [:graph:], but includes the space character
/[[:punct:]]/ - Punctuation character
/[[:space:]]/ - Whitespace character ([:blank:], newline, carriage return, etc.)
/[[:upper:]]/ - Uppercase alphabetical
/[[:xdigit:]]/ - Digit allowed in a hexadecimal number (i.e., 0-9a-fA-F)
Ruby also supports the following non-POSIX character classes:
/[[:word:]]/ - A character in one of the following Unicode general categories Letter, Mark, Number, Connector_Punctuation
/[[:ascii:]]/ - A character in the ASCII character set
# U+06F2 is "EXTENDED ARABIC-INDIC DIGIT TWO" /[[:digit:]]/.match("\u06F2") #=> #<MatchData "\u{06F2}"> /[[:upper:]][[:lower:]]/.match("Hello") #=> #<MatchData "He"> /[[:xdigit:]][[:xdigit:]]/.match("A6") #=> #<MatchData "A6">
The constructs described so far match a single character. They can be followed by a repetition metacharacter to specify how many times they need to occur. Such metacharacters are called quantifiers.
* - Zero or more times
+ - One or more times
? - Zero or one times (optional)
{n} - Exactly n times
{n,} - n or more times
{,m} - m or less times
{n,m} - At least n and at most m times
At least one uppercase character (‘H’), at least one lowercase character (‘e’), two ‘l’ characters, then one ‘o’:
"Hello".match(/[[:upper:]]+[[:lower:]]+l{2}o/) #=> #<MatchData "Hello">
Repetition is greedy by default: as many occurrences as possible are matched while still allowing the overall match to succeed. By contrast, lazy matching makes the minimal amount of matches necessary for overall success. Most greedy metacharacters can be made lazy by following them with ?. For the {n} pattern, because it specifies an exact number of characters to match and not a variable number of characters, the ? metacharacter instead makes the repeated pattern optional.
Both patterns below match the string. The first uses a greedy quantifier so ‘.+’ matches ‘<a><b>’; the second uses a lazy quantifier so ‘.+?’ matches ‘<a>’:
/<.+>/.match("<a><b>") #=> #<MatchData "<a><b>"> /<.+?>/.match("<a><b>") #=> #<MatchData "<a>">
A quantifier followed by + matches possessively: once it has matched it does not backtrack. They behave like greedy quantifiers, but having matched they refuse to “give up” their match even if this jeopardises the overall match.
/<.*><.+>/.match("<a><b>") #=> #<MatchData "<a><b>"> /<.*+><.+>/.match("<a><b>") #=> nil /<.*><.++>/.match("<a><b>") #=> nil
Parentheses can be used for capturing. The text enclosed by the nth group of parentheses can be subsequently referred to with n. Within a pattern use the backreference \n (e.g. \1); outside of the pattern use MatchData[n] (e.g. MatchData[1]).
In this example, 'at' is captured by the first group of parentheses, then referred to later with \1:
/[csh](..) [csh]\1 in/.match("The cat sat in the hat") #=> #<MatchData "cat sat in" 1:"at">
Regexp#match returns a MatchData object which makes the captured text available with its [] method:
/[csh](..) [csh]\1 in/.match("The cat sat in the hat")[1] #=> 'at'
While Ruby supports an arbitrary number of numbered captured groups, only groups 1-9 are supported using the \n backreference syntax.
Ruby also supports \0 as a special backreference, which references the entire matched string. This is also available at MatchData[0]. Note that the \0 backreference cannot be used inside the regexp, as backreferences can only be used after the end of the capture group, and the \0 backreference uses the implicit capture group of the entire match. However, you can use this backreference when doing substitution:
"The cat sat in the hat".gsub(/[csh]at/, '\0s') # => "The cats sats in the hats"
Capture groups can be referred to by name when defined with the (?<name>) or (?'name') constructs.
/\$(?<dollars>\d+)\.(?<cents>\d+)/.match("$3.67") #=> #<MatchData "$3.67" dollars:"3" cents:"67"> /\$(?<dollars>\d+)\.(?<cents>\d+)/.match("$3.67")[:dollars] #=> "3"
Named groups can be backreferenced with \k<name>, where name is the group name.
/(?<vowel>[aeiou]).\k<vowel>.\k<vowel>/.match('ototomy') #=> #<MatchData "ototo" vowel:"o">
Note: A regexp can’t use named backreferences and numbered backreferences simultaneously. Also, if a named capture is used in a regexp, then parentheses used for grouping which would otherwise result in a unnamed capture are treated as non-capturing.
/(\w)(\w)/.match("ab").captures # => ["a", "b"] /(\w)(\w)/.match("ab").named_captures # => {} /(?<c>\w)(\w)/.match("ab").captures # => ["a"] /(?<c>\w)(\w)/.match("ab").named_captures # => {"c"=>"a"}
When named capture groups are used with a literal regexp on the left-hand side of an expression and the =~ operator, the captured text is also assigned to local variables with corresponding names.
/\$(?<dollars>\d+)\.(?<cents>\d+)/ =~ "$3.67" #=> 0 dollars #=> "3"
Parentheses also group the terms they enclose, allowing them to be quantified as one atomic whole.
The pattern below matches a vowel followed by 2 word characters:
/[aeiou]\w{2}/.match("Caenorhabditis elegans") #=> #<MatchData "aen">
Whereas the following pattern matches a vowel followed by a word character, twice, i.e. [aeiou]\w[aeiou]\w: ‘enor’.
/([aeiou]\w){2}/.match("Caenorhabditis elegans") #=> #<MatchData "enor" 1:"or">
The (?:…) construct provides grouping without capturing. That is, it combines the terms it contains into an atomic whole without creating a backreference. This benefits performance at the slight expense of readability.
The first group of parentheses captures ‘n’ and the second ‘ti’. The second group is referred to later with the backreference \2:
/I(n)ves(ti)ga\2ons/.match("Investigations") #=> #<MatchData "Investigations" 1:"n" 2:"ti">
The first group of parentheses is now made non-capturing with ‘?:’, so it still matches ‘n’, but doesn’t create the backreference. Thus, the backreference \1 now refers to ‘ti’.
/I(?:n)ves(ti)ga\1ons/.match("Investigations") #=> #<MatchData "Investigations" 1:"ti">
Grouping can be made atomic with (?>pat). This causes the subexpression pat to be matched independently of the rest of the expression such that what it matches becomes fixed for the remainder of the match, unless the entire subexpression must be abandoned and subsequently revisited. In this way pat is treated as a non-divisible whole. Atomic grouping is typically used to optimise patterns so as to prevent the regular expression engine from backtracking needlessly.
The " in the pattern below matches the first character of the string, then .* matches Quote“. This causes the overall match to fail, so the text matched by .* is backtracked by one position, which leaves the final character of the string available to match "
/".*"/.match('"Quote"') #=> #<MatchData "\"Quote\"">
If .* is grouped atomically, it refuses to backtrack Quote“, even though this means that the overall match fails
/"(?>.*)"/.match('"Quote"') #=> nil
The \g<name> syntax matches the previous subexpression named name, which can be a group name or number, again. This differs from backreferences in that it re-executes the group rather than simply trying to re-match the same text.
This pattern matches a ( character and assigns it to the paren group, tries to call that the paren sub-expression again but fails, then matches a literal ):
/\A(?<paren>\(\g<paren>*\))*\z/ =~ '()' /\A(?<paren>\(\g<paren>*\))*\z/ =~ '(())' #=> 0 # ^1 # ^2 # ^3 # ^4 # ^5 # ^6 # ^7 # ^8 # ^9 # ^10
Matches at the beginning of the string, i.e. before the first character.
Enters a named capture group called paren
Matches a literal (, the first character in the string
Calls the paren group again, i.e. recurses back to the second step
Re-enters the paren group
Matches a literal (, the second character in the string
Try to call paren a third time, but fail because doing so would prevent an overall successful match
Match a literal ), the third character in the string. Marks the end of the second recursive call
Match a literal ), the fourth character in the string
Match the end of the string
The vertical bar metacharacter (|) combines several expressions into a single one that matches any of the expressions. Each expression is an alternative.
/\w(and|or)\w/.match("Feliformia") #=> #<MatchData "form" 1:"or"> /\w(and|or)\w/.match("furandi") #=> #<MatchData "randi" 1:"and"> /\w(and|or)\w/.match("dissemblance") #=> nil
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{Emoji}/ - Unicode emoji
/\p{Graph}/ - Non-blank character (excludes spaces, control characters, and similar)
/\p{Lower}/ - Lowercase alphabetical character
/\p{Print}/ - Like \p{Graph}, but includes the space character
/\p{Punct}/ - Punctuation character
/\p{Space}/ - Whitespace character ([:blank:], newline, carriage return, etc.)
/\p{Upper}/ - Uppercase alphabetical
/\p{XDigit}/ - Digit allowed in a hexadecimal number (i.e., 0-9a-fA-F)
/\p{Word}/ - A member of one of the following Unicode general category Letter, Mark, Number, Connector_Punctuation
/\p{ASCII}/ - A character in the ASCII character set
/\p{Any}/ - Any Unicode character (including unassigned characters)
/\p{Assigned}/ - An assigned character
A Unicode character’s General Category value can also be matched with \p{Ab} where Ab is the category’s abbreviation as described below:
/\p{L}/ - ‘Letter’
/\p{Ll}/ - ‘Letter: Lowercase’
/\p{Lm}/ - ‘Letter: Mark’
/\p{Lo}/ - ‘Letter: Other’
/\p{Lt}/ - ‘Letter: Titlecase’
/\p{Lu}/ - ‘Letter: Uppercase
/\p{Lo}/ - ‘Letter: Other’
/\p{M}/ - ‘Mark’
/\p{Mn}/ - ‘Mark: Nonspacing’
/\p{Mc}/ - ‘Mark: Spacing Combining’
/\p{Me}/ - ‘Mark: Enclosing’
/\p{N}/ - ‘Number’
/\p{Nd}/ - ‘Number: Decimal Digit’
/\p{Nl}/ - ‘Number: Letter’
/\p{No}/ - ‘Number: Other’
/\p{P}/ - ‘Punctuation’
/\p{Pc}/ - ‘Punctuation: Connector’
/\p{Pd}/ - ‘Punctuation: Dash’
/\p{Ps}/ - ‘Punctuation: Open’
/\p{Pe}/ - ‘Punctuation: Close’
/\p{Pi}/ - ‘Punctuation: Initial Quote’
/\p{Pf}/ - ‘Punctuation: Final Quote’
/\p{Po}/ - ‘Punctuation: Other’
/\p{S}/ - ‘Symbol’
/\p{Sm}/ - ‘Symbol: Math’
/\p{Sc}/ - ‘Symbol: Currency’
/\p{Sc}/ - ‘Symbol: Currency’
/\p{Sk}/ - ‘Symbol: Modifier’
/\p{So}/ - ‘Symbol: Other’
/\p{Z}/ - ‘Separator’
/\p{Zs}/ - ‘Separator: Space’
/\p{Zl}/ - ‘Separator: Line’
/\p{Zp}/ - ‘Separator: Paragraph’
/\p{C}/ - ‘Other’
/\p{Cc}/ - ‘Other: Control’
/\p{Cf}/ - ‘Other: Format’
/\p{Cn}/ - ‘Other: Not Assigned’
/\p{Co}/ - ‘Other: Private Use’
/\p{Cs}/ - ‘Other: Surrogate’
Lastly, \p{} matches a character’s Unicode script. The following scripts are supported: Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian, Cham, Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic, Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Inherited, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao, Latin, Lepcha, Limbu, Linear_B, Lycian, Lydian, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Ol_Chiki, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician, Rejang, Runic, Saurashtra, Shavian, Sinhala, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Vai, and Yi.
Unicode codepoint U+06E9 is named “ARABIC PLACE OF SAJDAH” and belongs to the Arabic script:
/\p{Arabic}/.match("\u06E9") #=> #<MatchData "\u06E9">
All character properties can be inverted by prefixing their name with a caret (^).
Letter ‘A’ is not in the Unicode Ll (Letter; Lowercase) category, so this match succeeds:
/\p{^Ll}/.match("A") #=> #<MatchData "A">
Anchors are metacharacter that match the zero-width positions between characters, anchoring the match to a specific position.
^ - Matches beginning of line
$ - Matches end of line
\A - Matches beginning of string.
\Z - Matches end of string. If string ends with a newline, it matches just before newline
\z - Matches end of string
\G - Matches first matching position:
In methods like String#gsub and String#scan, it changes on each iteration. It initially matches the beginning of subject, and in each following iteration it matches where the last match finished.
" a b c".gsub(/ /, '_') #=> "____a_b_c" " a b c".gsub(/\G /, '_') #=> "____a b c"
In methods like Regexp#match and String#match that take an (optional) offset, it matches where the search begins.
"hello, world".match(/,/, 3) #=> #<MatchData ","> "hello, world".match(/\G,/, 3) #=> nil
\b - Matches word boundaries when outside brackets; backspace (0x08) when inside brackets
\B - Matches non-word boundaries
(?=pat) - Positive lookahead assertion: ensures that the following characters match pat, but doesn’t include those characters in the matched text
(?!pat) - Negative lookahead assertion: ensures that the following characters do not match pat, but doesn’t include those characters in the matched text
(?<=pat) - Positive lookbehind assertion: ensures that the preceding characters match pat, but doesn’t include those characters in the matched text
(?<!pat) - Negative lookbehind assertion: ensures that the preceding characters do not match pat, but doesn’t include those characters in the matched text
\K - 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/ =~ "abc" #=> 0 /(?<=ab)c/ =~ "abc" #=> 2
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/
If a pattern isn’t anchored it can begin at any point in the string:
/real/.match("surrealist") #=> #<MatchData "real">
Anchoring the pattern to the beginning of the string forces the match to start there. ‘real’ doesn’t occur at the beginning of the string, so now the match fails:
/\Areal/.match("surrealist") #=> nil
The match below fails because although ‘Demand’ contains ‘and’, the pattern does not occur at a word boundary.
/\band/.match("Demand")
Whereas in the following example ‘and’ has been anchored to a non-word boundary so instead of matching the first ‘and’ it matches from the fourth letter of ‘demand’ instead:
/\Band.+/.match("Supply and demand curve") #=> #<MatchData "and curve">
The pattern below uses positive lookahead and positive lookbehind to match text appearing in tags without including the tags in the match:
/(?<=<b>)\w+(?=<\/b>)/.match("Fortune favours the <b>bold</b>") #=> #<MatchData "bold">
The end delimiter for a regexp can be followed by one or more single-letter options which control how the pattern can match.
/pat/i - Ignore case
/pat/m - Treat a newline as a character matched by .
/pat/x - Ignore whitespace and comments in the pattern
/pat/o - Perform #{} interpolation only once
i, m, and x can also be applied on the subexpression level with the (?on-off) construct, which enables options on, and disables options off for the expression enclosed by the parentheses:
/a(?i:b)c/.match('aBc') #=> #<MatchData "aBc"> /a(?-i:b)c/i.match('ABC') #=> nil
Additionally, these options can also be toggled for the remainder of the pattern:
/a(?i)bc/.match('abC') #=> #<MatchData "abC">
Options may also be used with Regexp.new:
Regexp.new("abc", Regexp::IGNORECASE) #=> /abc/i Regexp.new("abc", Regexp::MULTILINE) #=> /abc/m Regexp.new("abc # Comment", Regexp::EXTENDED) #=> /abc # Comment/x Regexp.new("abc", Regexp::IGNORECASE | Regexp::MULTILINE) #=> /abc/mi Regexp.new("abc", "i") #=> /abc/i Regexp.new("abc", "m") #=> /abc/m Regexp.new("abc # Comment", "x") #=> /abc # Comment/x Regexp.new("abc", "im") #=> /abc/mi
As mentioned above, the x option enables free-spacing mode. Literal white space inside the pattern is ignored, and the octothorpe (#) character introduces a comment until the end of the line. This allows the components of the pattern to be organized in a potentially more readable fashion.
A contrived pattern to match a number with optional decimal places:
float_pat = /\A [[:digit:]]+ # 1 or more digits before the decimal point (\. # Decimal point [[:digit:]]+ # 1 or more digits after the decimal point )? # The decimal point and following digits are optional \Z/x float_pat.match('3.14') #=> #<MatchData "3.14" 1:".14">
There are a number of strategies for matching whitespace:
Use a pattern such as \s or \p{Space}.
Use escaped whitespace such as \ , i.e. a space preceded by a backslash.
Use a character class such as [ ].
Comments can be included in a non-x pattern with the (?#comment) construct, where comment is arbitrary text ignored by the regexp engine.
Comments in regexp literals cannot include unescaped terminator characters.
Encoding Regular expressions are assumed to use the source encoding. This can be overridden with one of the following modifiers.
/pat/u - UTF-8
/pat/e - EUC-JP
/pat/s - Windows-31J
/pat/n - ASCII-8BIT
A regexp can be matched against a string when they either share an encoding, or the regexp’s encoding is US-ASCII and the string’s encoding is ASCII-compatible.
If a match between incompatible encodings is attempted an Encoding::CompatibilityError exception is raised.
The Regexp#fixed_encoding? predicate indicates whether the regexp has a fixed encoding, that is one incompatible with ASCII. A regexp’s encoding can be explicitly fixed by supplying Regexp::FIXEDENCODING as the second argument of Regexp.new:
r = Regexp.new("a".force_encoding("iso-8859-1"),Regexp::FIXEDENCODING) r =~ "a\u3042" # raises Encoding::CompatibilityError: incompatible encoding regexp match # (ISO-8859-1 regexp with UTF-8 string)
Pattern matching sets some global variables :
$~ is equivalent to Regexp.last_match;
$& contains the complete matched text;
$` contains string before match;
$' contains string after match;
$1, $2 and so on contain text matching first, second, etc capture group;
$+ contains last capture group.
Example:
m = /s(\w{2}).*(c)/.match('haystack') #=> #<MatchData "stac" 1:"ta" 2:"c"> $~ #=> #<MatchData "stac" 1:"ta" 2:"c"> Regexp.last_match #=> #<MatchData "stac" 1:"ta" 2:"c"> $& #=> "stac" # same as m[0] $` #=> "hay" # same as m.pre_match $' #=> "k" # same as m.post_match $1 #=> "ta" # same as m[1] $2 #=> "c" # same as m[2] $3 #=> nil # no third group in pattern $+ #=> "c" # same as m[-1]
These global variables are thread-local and method-local variables.
Certain pathological combinations of constructs can lead to abysmally bad performance.
Consider a string of 25 as, a d, 4 as, and a c.
s = 'a' * 25 + 'd' + 'a' * 4 + 'c' #=> "aaaaaaaaaaaaaaaaaaaaaaaaadaaaac"
The following patterns match instantly as you would expect:
/(b|a)/ =~ s #=> 0 /(b|a+)/ =~ s #=> 0 /(b|a+)*/ =~ s #=> 0
However, the following pattern takes appreciably longer:
/(b|a+)*c/ =~ s #=> 26
This happens because an atom in the regexp is quantified by both an immediate + and an enclosing * with nothing to differentiate which is in control of any particular character. The nondeterminism that results produces super-linear performance. (Consult Mastering Regular Expressions (3rd ed.), pp 222, by Jeffery Friedl, for an in-depth analysis). This particular case can be fixed by use of atomic grouping, which prevents the unnecessary backtracking:
(start = Time.now) && /(b|a+)*c/ =~ s && (Time.now - start) #=> 24.702736882 (start = Time.now) && /(?>b|a+)*c/ =~ s && (Time.now - start) #=> 0.000166571
A similar case is typified by the following example, which takes approximately 60 seconds to execute for me:
Match a string of 29 as against a pattern of 29 optional as followed by 29 mandatory as:
Regexp.new('a?' * 29 + 'a' * 29) =~ 'a' * 29
The 29 optional as match the string, but this prevents the 29 mandatory as that follow from matching. Ruby must then backtrack repeatedly so as to satisfy as many of the optional matches as it can while still matching the mandatory 29. It is plain to us that none of the optional matches can succeed, but this fact unfortunately eludes Ruby.
The best way to improve performance is to significantly reduce the amount of backtracking needed. For this case, instead of individually matching 29 optional as, a range of optional as can be matched all at once with a{0,29}:
Regexp.new('a{0,29}' + 'a' * 29) =~ 'a' * 29
Timeout There are two APIs to set timeout. One is Regexp.timeout=, which is process-global configuration of timeout for Regexp matching.
Regexp.timeout = 3 s = 'a' * 25 + 'd' + 'a' * 4 + 'c' /(b|a+)*c/ =~ s #=> This raises an exception in three seconds
The other is timeout keyword of Regexp.new.
re = Regexp.new("(b|a+)*c", timeout: 3) s = 'a' * 25 + 'd' + 'a' * 4 + 'c' /(b|a+)*c/ =~ s #=> This raises an exception in three seconds
When using Regexps to process untrusted input, you should use the timeout feature to avoid excessive backtracking. Otherwise, a malicious user can provide input to Regexp causing Denial-of-Service attack. Note that the timeout is not set by default because an appropriate limit highly depends on an application requirement and context.
Class Struct provides a convenient way to create a simple class that can store and fetch values.
This example creates a subclass of Struct, Struct::Customer; the first argument, a string, is the name of the subclass; the other arguments, symbols, determine the members of the new subclass.
Customer = Struct.new('Customer', :name, :address, :zip) Customer.name # => "Struct::Customer" Customer.class # => Class Customer.superclass # => Struct
Corresponding to each member are two methods, a writer and a reader, that store and fetch values:
methods = Customer.instance_methods false methods # => [:zip, :address=, :zip=, :address, :name, :name=]
An instance of the subclass may be created, and its members assigned values, via method ::new:
joe = Customer.new("Joe Smith", "123 Maple, Anytown NC", 12345) joe # => #<struct Struct::Customer name="Joe Smith", address="123 Maple, Anytown NC", zip=12345>
The member values may be managed thus:
joe.name # => "Joe Smith" joe.name = 'Joseph Smith' joe.name # => "Joseph Smith"
And thus; note that member name may be expressed as either a string or a symbol:
joe[:name] # => "Joseph Smith" joe[:name] = 'Joseph Smith, Jr.' joe['name'] # => "Joseph Smith, Jr."
See Struct::new.
First, what’s elsewhere. Class Struct:
Inherits from class Object.
Includes module Enumerable, which provides dozens of additional methods.
See also Data, which is a somewhat similar, but stricter concept for defining immutable value objects.
Here, class Struct provides methods that are useful for:
Struct Subclass ::new: Returns a new subclass of Struct.
==: Returns whether a given object is equal to self, using == to compare member values.
eql?: Returns whether a given object is equal to self, using eql? to compare member values.
[]: Returns the value associated with a given member name.
to_a, values, deconstruct: Returns the member values in self as an array.
deconstruct_keys: Returns a hash of the name/value pairs for given member names.
dig: Returns the object in nested objects that is specified by a given member name and additional arguments.
members: Returns an array of the member names.
select, filter: Returns an array of member values from self, as selected by the given block.
values_at: Returns an array containing values for given member names.
[]=: Assigns a given value to a given member name.
each: Calls a given block with each member name.
each_pair: Calls a given block with each member name/value pair.
SocketError is the error class for socket.
TCPServer represents a TCP/IP server socket.
A simple TCP server may look like:
require 'socket' server = TCPServer.new 2000 # Server bind to port 2000 loop do client = server.accept # Wait for a client to connect client.puts "Hello !" client.puts "Time is #{Time.now}" client.close end
A more usable server (serving multiple clients):
require 'socket' server = TCPServer.new 2000 loop do Thread.start(server.accept) do |client| client.puts "Hello !" client.puts "Time is #{Time.now}" client.close end end
TCPSocket represents a TCP/IP client socket.
A simple client may look like:
require 'socket' s = TCPSocket.new 'localhost', 2000 while line = s.gets # Read lines from socket puts line # and print them end s.close # close socket when done
IO streams for strings, with access similar to IO; see IO.
Examples on this page assume that StringIO has been required:
require 'stringio'