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Numeric

Numeric is the class from which all higher-level numeric classes should inherit.

Numeric allows instantiation of heap-allocated objects. Other core numeric classes such as Integer are implemented as immediates, which means that each Integer is a single immutable object which is always passed by value.

a = 1
1.object_id == a.object_id   #=> true

There can only ever be one instance of the integer 1, for example. Ruby ensures this by preventing instantiation. If duplication is attempted, the same instance is returned.

Integer.new(1)                   #=> NoMethodError: undefined method `new' for Integer:Class
1.dup                            #=> 1
1.object_id == 1.dup.object_id   #=> true

For this reason, Numeric should be used when defining other numeric classes.

Classes which inherit from Numeric must implement coerce, which returns a two-member Array containing an object that has been coerced into an instance of the new class and self (see coerce).

Inheriting classes should also implement arithmetic operator methods (+, -, * and /) and the <=> operator (see Comparable). These methods may rely on coerce to ensure interoperability with instances of other numeric classes.

class Tally < Numeric
  def initialize(string)
    @string = string
  end

  def to_s
    @string
  end

  def to_i
    @string.size
  end

  def coerce(other)
    [self.class.new('|' * other.to_i), self]
  end

  def <=>(other)
    to_i <=> other.to_i
  end

  def +(other)
    self.class.new('|' * (to_i + other.to_i))
  end

  def -(other)
    self.class.new('|' * (to_i - other.to_i))
  end

  def *(other)
    self.class.new('|' * (to_i * other.to_i))
  end

  def /(other)
    self.class.new('|' * (to_i / other.to_i))
  end
end

tally = Tally.new('||')
puts tally * 2            #=> "||||"
puts tally > 1            #=> true

What’s Here

First, what’s elsewhere. Class Numeric:

Here, class Numeric provides methods for:

Querying

Comparing

Converting

Other

Float

A Float object represents a sometimes-inexact real number using the native architecture’s double-precision floating point representation.

Floating point has a different arithmetic and is an inexact number. So you should know its esoteric system. See following:

You can create a Float object explicitly with:

You can convert certain objects to Floats with:

What’s Here

First, what’s elsewhere. Class Float:

Here, class Float provides methods for:

Querying

Comparing

Converting

Fiber

Fibers are primitives for implementing light weight cooperative concurrency in Ruby. Basically they are a means of creating code blocks that can be paused and resumed, much like threads. The main difference is that they are never preempted and that the scheduling must be done by the programmer and not the VM.

As opposed to other stackless light weight concurrency models, each fiber comes with a stack. This enables the fiber to be paused from deeply nested function calls within the fiber block. See the ruby(1) manpage to configure the size of the fiber stack(s).

When a fiber is created it will not run automatically. Rather it must be explicitly asked to run using the Fiber#resume method. The code running inside the fiber can give up control by calling Fiber.yield in which case it yields control back to caller (the caller of the Fiber#resume).

Upon yielding or termination the Fiber returns the value of the last executed expression

For instance:

fiber = Fiber.new do
  Fiber.yield 1
  2
end

puts fiber.resume
puts fiber.resume
puts fiber.resume

produces

1
2
FiberError: dead fiber called

The Fiber#resume method accepts an arbitrary number of parameters, if it is the first call to resume then they will be passed as block arguments. Otherwise they will be the return value of the call to Fiber.yield

Example:

fiber = Fiber.new do |first|
  second = Fiber.yield first + 2
end

puts fiber.resume 10
puts fiber.resume 1_000_000
puts fiber.resume "The fiber will be dead before I can cause trouble"

produces

12
1000000
FiberError: dead fiber called

Non-blocking Fibers

The concept of non-blocking fiber was introduced in Ruby 3.0. A non-blocking fiber, when reaching a operation that would normally block the fiber (like sleep, or wait for another process or I/O) will yield control to other fibers and allow the scheduler to handle blocking and waking up (resuming) this fiber when it can proceed.

For a Fiber to behave as non-blocking, it need to be created in Fiber.new with blocking: false (which is the default), and Fiber.scheduler should be set with Fiber.set_scheduler. If Fiber.scheduler is not set in the current thread, blocking and non-blocking fibers’ behavior is identical.

Ruby doesn’t provide a scheduler class: it is expected to be implemented by the user and correspond to Fiber::Scheduler.

There is also Fiber.schedule method, which is expected to immediately perform the given block in a non-blocking manner. Its actual implementation is up to the scheduler.

Enumerator

A class which allows both internal and external iteration.

An Enumerator can be created by the following methods.

Most methods have two forms: a block form where the contents are evaluated for each item in the enumeration, and a non-block form which returns a new Enumerator wrapping the iteration.

enumerator = %w(one two three).each
puts enumerator.class # => Enumerator

enumerator.each_with_object("foo") do |item, obj|
  puts "#{obj}: #{item}"
end

# foo: one
# foo: two
# foo: three

enum_with_obj = enumerator.each_with_object("foo")
puts enum_with_obj.class # => Enumerator

enum_with_obj.each do |item, obj|
  puts "#{obj}: #{item}"
end

# foo: one
# foo: two
# foo: three

This allows you to chain Enumerators together. For example, you can map a list’s elements to strings containing the index and the element as a string via:

puts %w[foo bar baz].map.with_index { |w, i| "#{i}:#{w}" }
# => ["0:foo", "1:bar", "2:baz"]

External Iteration

An Enumerator can also be used as an external iterator. For example, Enumerator#next returns the next value of the iterator or raises StopIteration if the Enumerator is at the end.

e = [1,2,3].each   # returns an enumerator object.
puts e.next   # => 1
puts e.next   # => 2
puts e.next   # => 3
puts e.next   # raises StopIteration

next, next_values, peek, and peek_values are the only methods which use external iteration (and Array#zip(Enumerable-not-Array) which uses next internally).

These methods do not affect other internal enumeration methods, unless the underlying iteration method itself has side-effect, e.g. IO#each_line.

FrozenError will be raised if these methods are called against a frozen enumerator. Since rewind and feed also change state for external iteration, these methods may raise FrozenError too.

External iteration differs significantly from internal iteration due to using a Fiber:

Concretely:

Thread.current[:fiber_local] = 1
Fiber[:storage_var] = 1
e = Enumerator.new do |y|
  p Thread.current[:fiber_local] # for external iteration: nil, for internal iteration: 1
  p Fiber[:storage_var] # => 1, inherited
  Fiber[:storage_var] += 1
  y << 42
end

p e.next # => 42
p Fiber[:storage_var] # => 1 (it ran in a different Fiber)

e.each { p _1 }
p Fiber[:storage_var] # => 2 (it ran in the same Fiber/"stack" as the current Fiber)

Convert External Iteration to Internal Iteration

You can use an external iterator to implement an internal iterator as follows:

def ext_each(e)
  while true
    begin
      vs = e.next_values
    rescue StopIteration
      return $!.result
    end
    y = yield(*vs)
    e.feed y
  end
end

o = Object.new

def o.each
  puts yield
  puts yield(1)
  puts yield(1, 2)
  3
end

# use o.each as an internal iterator directly.
puts o.each {|*x| puts x; [:b, *x] }
# => [], [:b], [1], [:b, 1], [1, 2], [:b, 1, 2], 3

# convert o.each to an external iterator for
# implementing an internal iterator.
puts ext_each(o.to_enum) {|*x| puts x; [:b, *x] }
# => [], [:b], [1], [:b, 1], [1, 2], [:b, 1, 2], 3

StopIteration

Raised to stop the iteration, in particular by Enumerator#next. It is rescued by Kernel#loop.

loop do
  puts "Hello"
  raise StopIteration
  puts "World"
end
puts "Done!"

produces:

Hello
Done!

Interrupt

Raised when the interrupt signal is received, typically because the user has pressed Control-C (on most posix platforms). As such, it is a subclass of SignalException.

begin
  puts "Press ctrl-C when you get bored"
  loop {}
rescue Interrupt => e
  puts "Note: You will typically use Signal.trap instead."
end

produces:

Press ctrl-C when you get bored

then waits until it is interrupted with Control-C and then prints:

Note: You will typically use Signal.trap instead.

StandardError

The most standard error types are subclasses of StandardError. A rescue clause without an explicit Exception class will rescue all StandardErrors (and only those).

def foo
  raise "Oups"
end
foo rescue "Hello"   #=> "Hello"

On the other hand:

require 'does/not/exist' rescue "Hi"

raises the exception:

LoadError: no such file to load -- does/not/exist

TypeError

Raised when encountering an object that is not of the expected type.

[1, 2, 3].first("two")

raises the exception:

TypeError: no implicit conversion of String into Integer

ArgumentError

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

IndexError

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

KeyError

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"

ScriptError

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).

SyntaxError

Raised when encountering Ruby code with an invalid syntax.

eval("1+1=2")

raises the exception:

SyntaxError: (eval):1: syntax error, unexpected '=', expecting $end

NotImplementedError

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.

NameError

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

NoMethodError

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 an instance of String

RuntimeError

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

FrozenError

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

SecurityError

No longer used by internal code.

NoMemoryError

Raised when memory allocation fails.

SystemCallError

SystemCallError is the base class for all low-level platform-dependent errors.

The errors available on the current platform are subclasses of SystemCallError and are defined in the Errno module.

File.open("does/not/exist")

raises the exception:

Errno::ENOENT: No such file or directory - does/not/exist

Range

A Range object represents a collection of values that are between given begin and end values.

You can create an Range object explicitly with:

A range may be created using method Range.new:

# Ranges that by default include the given end value.
Range.new(1, 4).to_a     # => [1, 2, 3, 4]
Range.new('a', 'd').to_a # => ["a", "b", "c", "d"]
# Ranges that use third argument +exclude_end+ to exclude the given end value.
Range.new(1, 4, true).to_a     # => [1, 2, 3]
Range.new('a', 'd', true).to_a # => ["a", "b", "c"]

Beginless Ranges

A beginless range has a definite end value, but a nil begin value. Such a range includes all values up to the end value.

r = (..4)               # => nil..4
r.begin                 # => nil
r.include?(-50)         # => true
r.include?(4)           # => true

r = (...4)              # => nil...4
r.include?(4)           # => false

Range.new(nil, 4)       # => nil..4
Range.new(nil, 4, true) # => nil...4

A beginless range may be used to slice an array:

a = [1, 2, 3, 4]
r = (..2) # => nil...2
a[r]      # => [1, 2]

Method each for a beginless range raises an exception.

Endless Ranges

An endless range has a definite begin value, but a nil end value. Such a range includes all values from the begin value.

r = (1..)         # => 1..
r.end             # => nil
r.include?(50)    # => true

Range.new(1, nil) # => 1..

The literal for an endless range may be written with either two dots or three. The range has the same elements, either way. But note that the two are not equal:

r0 = (1..)           # => 1..
r1 = (1...)          # => 1...
r0.begin == r1.begin # => true
r0.end == r1.end     # => true
r0 == r1             # => false

An endless range may be used to slice an array:

a = [1, 2, 3, 4]
r = (2..) # => 2..
a[r]      # => [3, 4]

Method each for an endless range calls the given block indefinitely:

a = []
r = (1..)
r.each do |i|
  a.push(i) if i.even?
  break if i > 10
end
a # => [2, 4, 6, 8, 10]

A range can be both beginless and endless. For literal beginless, endless ranges, at least the beginning or end of the range must be given as an explicit nil value. It is recommended to use an explicit nil beginning and implicit nil end, since that is what Ruby uses for Range#inspect:

(nil..)    # => (nil..)
(..nil)    # => (nil..)
(nil..nil) # => (nil..)

Ranges and Other Classes

An object may be put into a range if its class implements instance method <=>. Ruby core classes that do so include Array, Complex, File::Stat, Float, Integer, Kernel, Module, Numeric, Rational, String, Symbol, and Time.

Example:

t0 = Time.now         # => 2021-09-19 09:22:48.4854986 -0500
t1 = Time.now         # => 2021-09-19 09:22:56.0365079 -0500
t2 = Time.now         # => 2021-09-19 09:23:08.5263283 -0500
(t0..t2).include?(t1) # => true
(t0..t1).include?(t2) # => false

A range can be iterated over only if its elements implement instance method succ. Ruby core classes that do so include Integer, String, and Symbol (but not the other classes mentioned above).

Iterator methods include:

Example:

a = []
(1..4).each {|i| a.push(i) }
a # => [1, 2, 3, 4]

Ranges and User-Defined Classes

A user-defined class that is to be used in a range must implement instance <=>; see Integer#<=>. To make iteration available, it must also implement instance method succ; see Integer#succ.

The class below implements both <=> and succ, and so can be used both to construct ranges and to iterate over them. Note that the Comparable module is included so the == method is defined in terms of <=>.

# Represent a string of 'X' characters.
class Xs
  include Comparable
  attr_accessor :length
  def initialize(n)
    @length = n
  end
  def succ
    Xs.new(@length + 1)
  end
  def <=>(other)
    @length <=> other.length
  end
  def to_s
    sprintf "%2d #{inspect}", @length
  end
  def inspect
    'X' * @length
  end
end

r = Xs.new(3)..Xs.new(6) #=> XXX..XXXXXX
r.to_a                   #=> [XXX, XXXX, XXXXX, XXXXXX]
r.include?(Xs.new(5))    #=> true
r.include?(Xs.new(7))    #=> false

What’s Here

First, what’s elsewhere. Class Range:

Here, class Range provides methods that are useful for:

Methods for Creating a Range

Methods for Querying

Methods for Comparing

Methods for Iterating

Methods for Converting

Methods for Working with JSON

To make these methods available:

require 'json/add/range'

Regexp

A regular expression (also called a regexp) is a match pattern (also simply called a pattern).

A common notation for a regexp uses enclosing slash characters:

/foo/

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

Regexp Objects

A regexp object has:

Creating a Regexp

A regular expression may be created with:

Method match

Each of the methods Regexp#match, String#match, and Symbol#match returns a MatchData object if a match was found, nil otherwise; each also sets global variables:

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

Operator =~

Each of the operators Regexp#=~, String#=~, and Symbol#=~ returns an integer offset if a match was found, nil otherwise; each also sets global variables:

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

Method match?

Each of the methods Regexp#match?, String#match?, and Symbol#match? returns true if a match was found, false otherwise; none sets global variables:

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

Global Variables

Certain regexp-oriented methods assign values to global variables:

The affected global variables are:

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 that Regexp#match?, String#match?, and Symbol#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 available subexpressions gives the regexp great power and flexibility:

Special Characters

Regexp special characters, called metacharacters, 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 "\\.">

Method Regexp.escape returns an escaped string:

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

Source Literals

The source literal largely behaves like a double-quoted string; see Regexp 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; see Shorthand Character Classes.

Character Classes

A character 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 character except 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:

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:

Lookaround Anchors

Lookahead anchors:

Lookbehind anchors:

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">

Match-Reset Anchor

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') # => nil
re.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">

Method Regexp.union provides a convenient way to construct a regexp with alternatives.

Quantifiers

A simple regexp matches one character:

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

An added quantifier specifies how many matches are required or allowed:

Greedy, Lazy, or Possessive Matching

Quantifier matching may be greedy, lazy, or possessive:

More:

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 # => 1
re.match('foo')             # => nil

Adding one or more pairs of parentheses, (subexpression), defines groups, which may result in multiple matched substrings, called captures:

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 a quantifier:

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('')       # => nil
re.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 a backreference:

/[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:

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 in MatchData[]:

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"

Method Regexp#named_captures returns a hash of the capture names and substrings; method Regexp#names returns an array of the capture names.

Atomic Grouping

A group may be made atomic 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 way subexpression 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 substring Quote“ (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; see Atomic Group.

Subexpression Calls

As seen above, a backreference number (\n) or name (\k<name>) gives access to a captured substring; the corresponding regexp subexpression 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 group paren.

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

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

  5. Re-enters the paren group.

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

  7. Attempts to call paren 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.

See Subexpression calls.

Conditionals

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

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') # => nil
re.match('T')    # => nil

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

Absence Operator

The absence operator is a special group that matches anything which does not 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 lowercase p) matches characters using a Unicode property name, much like a character class; property Alpha 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 (uppercase P):

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

See Unicode Properties for regexps based on the numerous properties.

Some commonly-used properties correspond to POSIX bracket expressions:

These are also commonly used:

Unicode Character Categories

A Unicode character category name:

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:

Marks:

Numbers:

Punctation:

Unicode Scripts and Blocks

Among the Unicode properties are:

POSIX Bracket Expressions

A POSIX bracket 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:

The POSIX bracket expressions:

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

Comments

A comment may be included in a regexp pattern using the (?#comment) construct, where comment 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 also Extended Mode.

Modes

Each of these modifiers sets a mode for the regexp:

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

Modifiers i, m, and x may be applied to subexpressions:

Example:

re = /(?i)te(?-i)st/
re.match('test') # => #<MatchData "test">
re.match('TEst') # => #<MatchData "TEst">
re.match('TEST') # => nil
re.match('teST') # => nil

re = /t(?i:e)st/
re.match('test') # => #<MatchData "test">
re.match('tEst') # => #<MatchData "tEst">
re.match('tEST') # => nil

Method Regexp#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

Modifier i enables case-insensitive mode:

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

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

Multiline Mode

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

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

Extended Mode

Modifier x enables extended mode, which means that:

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 string
EOT
re = /#{pattern}/x
re.match('MCMXLIII') # => #<MatchData "MCMXLIII" 1:"CM" 2:"XL" 3:"III">

Interpolation Mode

Modifier o means that the first time a literal regexp with interpolations is encountered, the generated Regexp object is saved and used for all future evaluations of that literal regexp. Without modifier o, the generated Regexp is not saved, so each evaluation of the literal regexp generates a new Regexp object.

Without modifier o:

def letters; sleep 5; /[A-Z][a-z]/; end
words = %w[abc def xyz]
start = Time.now
words.each {|word| word.match(/\A[#{letters}]+\z/) }
Time.now - start # => 15.0174892

With modifier o:

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

Note that if the literal regexp does not have interpolations, the o 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.

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

If a match between incompatible encodings is attempted an Encoding::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") # => 0
re =~ "foo\u0100"           # Raises Encoding::CompatibilityError

The encoding may be explicitly fixed by including Regexp::FIXEDENCODING in the second argument for Regexp.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:

When regexp.timeout is nil, the timeout “falls through” to Regexp.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 the regular 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 attach is not possible.

This optimization is applied if the pattern meets these criteria:

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

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

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

References

Read (online PDF books):

Explore, test (interactive online editor):

Ripper

Ripper is a Ruby script parser.

You can get information from the parser with event-based style. Information such as abstract syntax trees or simple lexical analysis of the Ruby program.

Usage

Ripper provides an easy interface for parsing your program into a symbolic expression tree (or S-expression).

Understanding the output of the parser may come as a challenge, it’s recommended you use PP to format the output for legibility.

require 'ripper'
require 'pp'

pp Ripper.sexp('def hello(world) "Hello, #{world}!"; end')
  #=> [:program,
       [[:def,
         [:@ident, "hello", [1, 4]],
         [:paren,
          [:params, [[:@ident, "world", [1, 10]]], nil, nil, nil, nil, nil, nil]],
         [:bodystmt,
          [[:string_literal,
            [:string_content,
             [:@tstring_content, "Hello, ", [1, 18]],
             [:string_embexpr, [[:var_ref, [:@ident, "world", [1, 27]]]]],
             [:@tstring_content, "!", [1, 33]]]]],
          nil,
          nil,
          nil]]]]

You can see in the example above, the expression starts with :program.

From here, a method definition at :def, followed by the method’s identifier :@ident. After the method’s identifier comes the parentheses :paren and the method parameters under :params.

Next is the method body, starting at :bodystmt (stmt meaning statement), which contains the full definition of the method.

In our case, we’re simply returning a String, so next we have the :string_literal expression.

Within our :string_literal you’ll notice two @tstring_content, this is the literal part for Hello, and !. Between the two @tstring_content statements is a :string_embexpr, where embexpr is an embedded expression. Our expression consists of a local variable, or var_ref, with the identifier (@ident) of world.

Resources

Requirements

License

Ruby License.

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