Too Expressive & * _ ?

Ruby's syntax is so expressive — it utilizes every printable, non-alphanumeric ASCII character as much as it cans. Sometimes, this can be confusing for beginners.

The next sections show 4+ different meanings of every portrayed single character (not counting different meaning as custom string delimiter or meaning within a regex):

Question Mark (4 Syntactical Meanings)

The question mark is a sure way to confuse your syntax highlighter!

Valid ending of a method name

[].empty? # => true

?: Part of the Ternary Operator

nil ? true : false # => false

? char literals

?? # => "?"

Part of Special Variable $?

system "ls"
$? #=> #<Process::Status: pid 5223 exit 0>

Underscore (5 Syntactical Meanings)

Underscores serve as a good example of having multiple uses: No purpose interferes with another.

Valid Part of an Identifier (and $_)

variable_name = 42

Part of Special Keywords (__END__, __FILE__, __caller__)

File.dirname(__FILE__)

Allowed to Visually Separate Numbers

1_000_000 # => 1000000

Ignored Parameter

# other names would raise a SyntaxError (duplicated argument name)
def method_name(_,_)
  p _
end

method_name(42, 23)
# -> 42

2.7: Numbered Parameter

def method_name(param)
  p _1
end

method_name(42)
# -> 42

Ampersand (6 Syntactical Meanings)

&& Method/Operator

p true && false
# => false

& Method/Operator for Numbers, Arrays, Booleans

# set operation "and"
[1, 2] & [2] # => 2

# bit operation "and"
2 & 3 # => 2

Explicit Block Param

def method_name(&block)
  p block.call(1)
  p block.call(2)
end

method_name{ |x| x.odd? }
# -> true
# -> false

to_proc Shortcut

class Class
  def to_proc
    lambda{ |*args| self.new(*args) }
  end
end

[[1,2],[3,5,6,7,3]].map(&Set) # => [Set[1,2], Set[5,6,7,3]]

Part of Special Variable $&

/sync/ =~ "Idiosyncratic"
$& # => "sync"

2.3: &. Safe Navigation Operator

nil&.blank? # => nil

Star (7 Syntactical Meanings)

The record holder!

.* Method/Operator for Numbers, Strings, Arrays

4 * 4 # => 16
["4", "2"] * "." # => "4.2"

.** Method/Operator for Numbers

4 ** 4 # => 256

* Rest Arguments

def method_name(*arguments)
  p arguments
end

method_name(42, 23)
# -> [42, 23]

* Splat Arguments

[1, *2..4] # => [1, 2, 3, 4]

** Keyword Rest Arguments

def method_name(**arguments)
  p arguments
end

method_name(a: 42, b: 23)
# -> { a: 42, b: 23 }

** Keyword Splat Arguments

hash1 = { a: 1, b: 2 }
hash2 = { c: 3, **hash1 } # => { a: 1, b: 2, c: 3 }

Part of Special Variable $*

$ ruby -e "p $*" -- 42 23
["42", "23"]

Also See

More Idiosyncratic Ruby