ruby/doc/syntax/literals.rdoc

= Literals

Literals create objects you can use in your program.  Literals include:

* {Boolean and Nil Literals}[#label-Boolean+and+Nil+Literals]
* {Number Literals}[#label-Number+Literals]

  * {Integer Literals}[#label-Integer+Literals]
  * {Float Literals}[#label-Float+Literals]
  * {Rational Literals}[#label-Rational+Literals]
  * {Complex Literals}[#label-Complex+Literals]

* {String Literals}[#label-String+Literals]
* {Here Document Literals}[#label-Here+Document+Literals]
* {Symbol Literals}[#label-Symbol+Literals]
* {Array Literals}[#label-Array+Literals]
* {Hash Literals}[#label-Hash+Literals]
* {Range Literals}[#label-Range+Literals]
* {Regexp Literals}[#label-Regexp+Literals]
* {Lambda Proc Literals}[#label-Lambda+Proc+Literals]
* {Percent Literals}[#label-Percent+Literals]

  * {%q: Non-Interpolable String Literals}[#label-25q-3A+Non-Interpolable+String+Literals]
  * {% and %Q: Interpolable String Literals}[#label-25+and+-25Q-3A+Interpolable+String+Literals]
  * {%w and %W: String-Array Literals}[#label-25w+and+-25W-3A+String-Array+Literals]
  * {%i and %I: Symbol-Array Literals}[#label-25i+and+-25I-3A+Symbol-Array+Literals]
  * {%r: Regexp Literals}[#label-25r-3A+Regexp+Literals]
  * {%s: Symbol Literals}[#label-25s-3A+Symbol+Literals]
  * {%x: Backtick Literals}[#label-25x-3A+Backtick+Literals]

== Boolean and Nil Literals

+nil+ and +false+ are both false values.  +nil+ is sometimes used to indicate
"no value" or "unknown" but evaluates to +false+ in conditional expressions.

+true+ is a true value.  All objects except +nil+ and +false+ evaluate to a
true value in conditional expressions.

== Number Literals

=== \Integer Literals

You can write integers of any size as follows:

  1234
  1_234

These numbers have the same value, 1,234.  The underscore may be used to
enhance readability for humans.  You may place an underscore anywhere in the
number.

You can use a special prefix to write numbers in decimal, hexadecimal, octal
or binary formats.  For decimal numbers use a prefix of <tt>0d</tt>, for
hexadecimal numbers use a prefix of <tt>0x</tt>, for octal numbers use a
prefix of <tt>0</tt> or <tt>0o</tt>, for binary numbers use a prefix of
<tt>0b</tt>.  The alphabetic component of the number is not case-sensitive.

Examples:

  0d170
  0D170

  0xaa
  0xAa
  0xAA
  0Xaa
  0XAa
  0XaA

  0252
  0o252
  0O252

  0b10101010
  0B10101010

All these numbers have the same decimal value, 170.  Like integers and floats
you may use an underscore for readability.

=== \Float Literals

Floating-point numbers may be written as follows:

  12.34
  1234e-2
  1.234E1

These numbers have the same value, 12.34.  You may use underscores in floating
point numbers as well.

=== \Rational Literals

You can write a Rational literal using a special suffix, <tt>'r'</tt>.

Examples:

  1r       # => (1/1)
  2/3r     # => (2/3)   # With denominator.
  -1r      # => (-1/1)  # With signs.
  -2/3r    # => (-2/3)
  2/-3r    # => (-2/3)
  -2/-3r   # => (2/3)
  +1/+3r   # => (1/3)
  1.2r     # => (6/5)   # With fractional part.
  1_1/2_1r # => (11/21) # With embedded underscores.
  2/4r     # => (1/2)   # Automatically reduced.

Syntax:

  <rational-literal> = <numerator> [ '/' <denominator> ] 'r'
  <numerator> = [ <sign> ] <digits> [ <fractional-part> ]
  <fractional-part> = '.' <digits>
  <denominator> = [ sign ] <digits>
  <sign> = '-' | '+'
  <digits> = <digit> { <digit> | '_' <digit> }
  <digit> = '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9'

Note this, which is parsed as \Float numerator <tt>1.2</tt>
divided by \Rational denominator <tt>3r</tt>,
resulting in a \Float:

  1.2/3r  # => 0.39999999999999997

=== \Complex Literals

You can write a Complex number as follows (suffixed +i+):

  1i          #=> (0+1i)
  1i * 1i     #=> (-1+0i)

Also \Rational numbers may be imaginary numbers.

  12.3ri      #=> (0+(123/10)*i)

+i+ must be placed after +r+; the opposite is not allowed.

  12.3ir      #=> Syntax error

== Strings

=== Escape Sequences

Some characters can be represented as escape sequences in
double-quoted strings,
character literals,
here document literals (non-quoted, double-quoted, and with backticks),
double-quoted symbols,
double-quoted symbol keys in Hash literals,
Regexp literals, and
several percent literals (<tt>%</tt>, <tt>%Q,</tt> <tt>%W</tt>, <tt>%I</tt>, <tt>%r</tt>, <tt>%x</tt>).

They allow escape sequences such as <tt>\n</tt> for
newline, <tt>\t</tt> for tab, etc.  The full list of supported escape
sequences are as follows:

  \a             bell, ASCII 07h (BEL)
  \b             backspace, ASCII 08h (BS)
  \t             horizontal tab, ASCII 09h (TAB)
  \n             newline (line feed), ASCII 0Ah (LF)
  \v             vertical tab, ASCII 0Bh (VT)
  \f             form feed, ASCII 0Ch (FF)
  \r             carriage return, ASCII 0Dh (CR)
  \e             escape, ASCII 1Bh (ESC)
  \s             space, ASCII 20h (SPC)
  \\             backslash, \
  \nnn           octal bit pattern, where nnn is 1-3 octal digits ([0-7])
  \xnn           hexadecimal bit pattern, where nn is 1-2 hexadecimal digits ([0-9a-fA-F])
  \unnnn         Unicode character, where nnnn is exactly 4 hexadecimal digits ([0-9a-fA-F])
  \u{nnnn ...}   Unicode character(s), where each nnnn is 1-6 hexadecimal digits ([0-9a-fA-F])
  \cx or \C-x    control character, where x is an ASCII printable character
  \M-x           meta character, where x is an ASCII printable character
  \M-\C-x        meta control character, where x is an ASCII printable character
  \M-\cx         same as above
  \c\M-x         same as above
  \c? or \C-?    delete, ASCII 7Fh (DEL)
  \<newline>     continuation line (empty string)

The last one, <tt>\<newline></tt>, represents an empty string instead of a character.
It is used to fold a line in a string.

=== Double-quoted \String Literals

The most common way of writing strings is using <tt>"</tt>:

  "This is a string."

The string may be many lines long.

Any internal <tt>"</tt> must be escaped:

  "This string has a quote: \".  As you can see, it is escaped"

Double-quoted strings allow escape sequences described in
{Escape Sequences}[#label-Escape+Sequences].

In a double-quoted string,
any other character following a backslash is interpreted as the
character itself.

Double-quoted strings allow interpolation of other values using
<tt>#{...}</tt>:

  "One plus one is two: #{1 + 1}"

Any expression may be placed inside the interpolated section, but it's best to
keep the expression small for readability.

You can also use <tt>#@foo</tt>, <tt>#@@foo</tt> and <tt>#$foo</tt> as a
shorthand for, respectively, <tt>#{ @foo }</tt>, <tt>#{ @@foo }</tt> and
<tt>#{ $foo }</tt>.

See also:

* {% and %Q: Interpolable String Literals}[#label-25+and+-25Q-3A+Interpolable+String+Literals]

=== Single-quoted \String Literals

Interpolation may be disabled by escaping the "#" character or using
single-quoted strings:

  '#{1 + 1}' #=> "\#{1 + 1}"

In addition to disabling interpolation, single-quoted strings also disable all
escape sequences except for the single-quote (<tt>\'</tt>) and backslash
(<tt>\\\\</tt>).

In a single-quoted string,
any other character following a backslash is interpreted as is:
a backslash and the character itself.

See also:

* {%q: Non-Interpolable String Literals}[#label-25q-3A+Non-Interpolable+String+Literals]

=== Literal String Concatenation

Adjacent string literals are automatically concatenated by the interpreter:

  "con" "cat" "en" "at" "ion" #=> "concatenation"
  "This string contains "\
  "no newlines."              #=> "This string contains no newlines."

Any combination of adjacent single-quote, double-quote, percent strings will
be concatenated as long as a percent-string is not last.

  %q{a} 'b' "c" #=> "abc"
  "a" 'b' %q{c} #=> NameError: uninitialized constant q

=== Character Literal

There is also a character literal notation to represent single
character strings, which syntax is a question mark (<tt>?</tt>)
followed by a single character or escape sequence (except continuation line)
that corresponds to a single codepoint in the script encoding:

  ?a       #=> "a"
  ?abc     #=> SyntaxError
  ?\n      #=> "\n"
  ?\s      #=> " "
  ?\\      #=> "\\"
  ?\u{41}  #=> "A"
  ?\C-a    #=> "\x01"
  ?\M-a    #=> "\xE1"
  ?\M-\C-a #=> "\x81"
  ?\C-\M-a #=> "\x81", same as above
  ?あ      #=> "あ"

=== Here Document Literals

If you are writing a large block of text you may use a "here document" or
"heredoc":

  expected_result = <<HEREDOC
  This would contain specially formatted text.

  That might span many lines
  HEREDOC

The heredoc starts on the line following <tt><<HEREDOC</tt> and ends with the
next line that starts with <tt>HEREDOC</tt>.  The result includes the ending
newline.

You may use any identifier with a heredoc, but all-uppercase identifiers are
typically used.

You may indent the ending identifier if you place a "-" after <tt><<</tt>:

    expected_result = <<-INDENTED_HEREDOC
  This would contain specially formatted text.

  That might span many lines
    INDENTED_HEREDOC

Note that while the closing identifier may be indented, the content is
always treated as if it is flush left.  If you indent the content those spaces
will appear in the output.

To have indented content as well as an indented closing identifier, you can use
a "squiggly" heredoc, which uses a "~" instead of a "-" after <tt><<</tt>:

    expected_result = <<~SQUIGGLY_HEREDOC
      This would contain specially formatted text.

      That might span many lines
    SQUIGGLY_HEREDOC

The indentation of the least-indented line will be removed from each line of
the content.  Note that empty lines and lines consisting solely of literal tabs
and spaces will be ignored for the purposes of determining indentation, but
escaped tabs and spaces are considered non-indentation characters.

For the purpose of measuring an indentation, a horizontal tab is regarded as a
sequence of one to eight spaces such that the column position corresponding to
its end is a multiple of eight.  The amount to be removed is counted in terms
of the number of spaces.  If the boundary appears in the middle of a tab, that
tab is not removed.

A heredoc allows interpolation and the escape sequences described in
{Escape Sequences}[#label-Escape+Sequences].
You may disable interpolation and the escaping by surrounding the opening
identifier with single quotes:

  expected_result = <<-'EXPECTED'
  One plus one is #{1 + 1}
  EXPECTED

  p expected_result # prints: "One plus one is \#{1 + 1}\n"

The identifier may also be surrounded with double quotes (which is the same as
no quotes) or with backticks.  When surrounded by backticks the HEREDOC
behaves like Kernel#`:

  puts <<-`HEREDOC`
  cat #{__FILE__}
  HEREDOC

When surrounding with quotes, any character but that quote and newline
(CR and/or LF) can be used as the identifier.

To call a method on a heredoc place it after the opening identifier:

  expected_result = <<-EXPECTED.chomp
  One plus one is #{1 + 1}
  EXPECTED

You may open multiple heredocs on the same line, but this can be difficult to
read:

  puts(<<-ONE, <<-TWO)
  content for heredoc one
  ONE
  content for heredoc two
  TWO

== \Symbol Literals

A Symbol represents a name inside the ruby interpreter.  See Symbol for more
details on what symbols are and when ruby creates them internally.

You may reference a symbol using a colon: <tt>:my_symbol</tt>.

You may also create symbols by interpolation and escape sequences described in
{Escape Sequences}[#label-Escape+Sequences] with double-quotes:

  :"my_symbol1"
  :"my_symbol#{1 + 1}"
  :"foo\sbar"

Like strings, a single-quote may be used to disable interpolation and
escape sequences:

  :'my_symbol#{1 + 1}' #=> :"my_symbol\#{1 + 1}"

When creating a Hash, there is a special syntax for referencing a Symbol as
well.

See also:

* {%s: Symbol Literals}[#label-25s-3A+Symbol+Literals]


== \Array Literals

An array is created using the objects between <tt>[</tt> and <tt>]</tt>:

  [1, 2, 3]

You may place expressions inside the array:

  [1, 1 + 1, 1 + 2]
  [1, [1 + 1, [1 + 2]]]

See also:

* {%w and %W: String-Array Literals}[#label-25w+and+-25W-3A+String-Array+Literals]
* {%i and %I: Symbol-Array Literals}[#label-25i+and+-25I-3A+Symbol-Array+Literals]

See Array for the methods you may use with an array.

== \Hash Literals

A hash is created using key-value pairs between <tt>{</tt> and <tt>}</tt>:

  { "a" => 1, "b" => 2 }

Both the key and value may be any object.

You can create a hash using symbol keys with the following syntax:

  { a: 1, b: 2 }

This same syntax is used for keyword arguments for a method.

Like Symbol literals, you can quote symbol keys.

  { "a 1": 1, "b #{1 + 1}": 2 }

is equal to

  { :"a 1" => 1, :"b 2" => 2 }

Hash values can be omitted, meaning that value will be fetched from the context
by the name of the key:

  x = 100
  y = 200
  h = { x:, y: }
  #=> {:x=>100, :y=>200}

See Hash for the methods you may use with a hash.

== \Range Literals

A range represents an interval of values.  The range may include or exclude
its ending value.

  (1..2)  # includes its ending value
  (1...2) # excludes its ending value
  (1..)   # endless range, representing infinite sequence from 1 to Infinity
  (..1)   # beginless range, representing infinite sequence from -Infinity to 1

You may create a range of any object.  See the Range documentation for details
on the methods you need to implement.

== \Regexp Literals

A regular expression may be created using leading and trailing
slash (<tt>'/'</tt>) characters:

  re = /foo/ # => /foo/
  re.class   # => Regexp

The trailing slash may be followed by one or more modifiers characters
that set modes for the regexp.
See {Regexp modes}[rdoc-ref:Regexp@Modes] for details.

Interpolation may be used inside regular expressions along with escaped
characters.  Note that a regular expression may require additional escaped
characters than a string.

See also:

* {%r: Regexp Literals}[#label-25r-3A+Regexp+Literals]

See Regexp for a description of the syntax of regular expressions.

== Lambda Proc Literals

A lambda proc can be created with <tt>-></tt>:

  -> { 1 + 1 }

Calling the above proc will give a result of <tt>2</tt>.

You can require arguments for the proc as follows:

  ->(v) { 1 + v }

This proc will add one to its argument.

== Percent Literals

Each of the literals in described in this section
may use these paired delimiters:

* <tt>[</tt> and <tt>]</tt>.
* <tt>(</tt> and <tt>)</tt>.
* <tt>{</tt> and <tt>}</tt>.
* <tt><</tt> and <tt>></tt>.
* Non-alphanumeric ASCII character except above, as both beginning and ending delimiters.

The delimiters can be escaped with a backslash.
However, the first four pairs (brackets, parenthesis, braces, and
angle brackets) are allowed without backslash as far as they are correctly
paired.

These are demonstrated in the next section.

=== <tt>%q</tt>: Non-Interpolable String Literals

You can write a non-interpolable string with <tt>%q</tt>.
The created string is the same as if you created it with single quotes:

  %q[foo bar baz]       # => "foo bar baz" # Using [].
  %q(foo bar baz)       # => "foo bar baz" # Using ().
  %q{foo bar baz}       # => "foo bar baz" # Using {}.
  %q<foo bar baz>       # => "foo bar baz" # Using <>.
  %q|foo bar baz|       # => "foo bar baz" # Using two |.
  %q:foo bar baz:       # => "foo bar baz" # Using two :.
  %q(1 + 1 is #{1 + 1}) # => "1 + 1 is \#{1 + 1}" # No interpolation.
  %q[foo[bar]baz]       # => "foo[bar]baz" # brackets can be nested.
  %q(foo(bar)baz)       # => "foo(bar)baz" # parenthesis can be nested.
  %q{foo{bar}baz}       # => "foo{bar}baz" # braces can be nested.
  %q<foo<bar>baz>       # => "foo<bar>baz" # angle brackets can be nested.

This is similar to single-quoted string but only backslashs and
the specified delimiters can be escaped with a backslash.

=== <tt>% and %Q</tt>: Interpolable String Literals

You can write an interpolable string with <tt>%Q</tt>
or with its alias <tt>%</tt>:

  %[foo bar baz]       # => "foo bar baz"
  %(1 + 1 is #{1 + 1}) # => "1 + 1 is 2" # Interpolation.

This is similar to double-quoted string.
It allow escape sequences described in
{Escape Sequences}[#label-Escape+Sequences].
Other escaped characters (a backslash followed by a character) are
interpreted as the character.

=== <tt>%w and %W</tt>: String-Array Literals

You can write an array of strings as whitespace-separated words
with <tt>%w</tt> (non-interpolable) or <tt>%W</tt> (interpolable):

  %w[foo bar baz]       # => ["foo", "bar", "baz"]
  %w[1 % *]             # => ["1", "%", "*"]
  # Use backslash to embed spaces in the strings.
  %w[foo\ bar baz\ bat] # => ["foo bar", "baz bat"]
  %W[foo\ bar baz\ bat] # => ["foo bar", "baz bat"]
  %w(#{1 + 1})          # => ["\#{1", "+", "1}"]
  %W(#{1 + 1})          # => ["2"]

  # The nested delimiters evaluated to a flat array of strings
  # (not nested array).
  %w[foo[bar baz]qux]   # => ["foo[bar", "baz]qux"]

The following characters are considered as white spaces to separate words:

* space, ASCII 20h (SPC)
* form feed, ASCII 0Ch (FF)
* newline (line feed), ASCII 0Ah (LF)
* carriage return, ASCII 0Dh (CR)
* horizontal tab, ASCII 09h (TAB)
* vertical tab, ASCII 0Bh (VT)

The white space characters can be escaped with a backslash to make them
part of a word.

<tt>%W</tt> allow escape sequences described in
{Escape Sequences}[#label-Escape+Sequences].
However the continuation line <tt>\<newline></tt> is not usable because
it is interpreted as the escaped newline described above.

=== <tt>%i and %I</tt>: Symbol-Array Literals

You can write an array of symbols as whitespace-separated words
with <tt>%i</tt> (non-interpolable) or <tt>%I</tt> (interpolable):

  %i[foo bar baz]       # => [:foo, :bar, :baz]
  %i[1 % *]             # => [:"1", :%, :*]
  # Use backslash to embed spaces in the symbols.
  %i[foo\ bar baz\ bat] # => [:"foo bar", :"baz bat"]
  %I[foo\ bar baz\ bat] # => [:"foo bar", :"baz bat"]
  %i(#{1 + 1})          # => [:"\#{1", :+, :"1}"]
  %I(#{1 + 1})          # => [:"2"]

The white space characters and its escapes are interpreted as the same as
string-array literals described in
{%w and %W: String-Array Literals}[#label-25w+and+-25W-3A+String-Array+Literals].

=== <tt>%s</tt>: Symbol Literals

You can write a symbol with <tt>%s</tt>:

  %s[foo]     # => :foo
  %s[foo bar] # => :"foo bar"

This is non-interpolable.
No interpolation allowed.
Only backslashs and the specified delimiters can be escaped with a backslash.

=== <tt>%r</tt>: Regexp Literals

You can write a regular expression with <tt>%r</tt>;
the character used as the leading and trailing delimiter
may be (almost) any character:

  %r/foo/             # => /foo/
  %r:name/value pair: # => /name\/value pair/

A few "symmetrical" character pairs may be used as delimiters:

  %r[foo] # => /foo/
  %r{foo} # => /foo/
  %r(foo) # => /foo/
  %r<foo> # => /foo/

The trailing delimiter may be followed by one or more modifier characters
that set modes for the regexp.
See {Regexp modes}[rdoc-ref:Regexp@Modes] for details.

=== <tt>%x</tt>: Backtick Literals

You can write and execute a shell command with <tt>%x</tt>:

  %x(echo 1)      # => "1\n"
  %x[echo #{1 + 2}] # => "3\n"
  %x[echo \u0030] # => "0\n"

This is interpolable.
<tt>%x</tt> allow escape sequences described in
{Escape Sequences}[#label-Escape+Sequences].