Question

I’m coming across the square bracket [] syntax quite a bit in Ruby, but it never seems to be doing the same thing. Can anyone list all the different uses for the square brackets [] in Ruby so my mind can get a handle on this seemingly endlessly versatile little symbol? (How is it possible that one symbol can do so much without the Ruby interpreter getting confused?)

Examples:

• [] and []= methods
• %q[...]
• [1,2,3][0]
• hash["a"] = 3
• ary = []
• /[^A-Fa-f0-9]/
• "Is a string"[5,3]

Answer 1

Okay, just for my own notes I have gone and had a closer look at this and, building on Holger Just’s answer, come up with the following: the use of square brackets in Ruby can be divided into 6 uses, 3 of them a part of Ruby’s method definitions and 3 of them semantic constructs.

Method definition

Object creation via class methods Array::[], Hash::[]

Array.[](1,2,3) #=> [1,2,3]                        #Un-sugared notation
Array["a","b","c"] #=> ["a","b","c"]               #Sugared equivalent
Hash["a", 100, "b", 200] #=> {"a"=>100, "b"=>200}  

Nothing to do with literal constructors, although it does the same thing.

Element reference via instance methods Array#[], Bignum#[], Continuation#[], Fixnum#[], Hash#[], MatchData#[], Method#[], Proc#[], String#[], Struct#[], Symbol#[], Thread#[], and class methods Dir::[], ENV::[]

ary = [1,2,"abc", [15,16,[26,27]]]  
ary.[](2) #=> "abc"                #Un-sugared notation
ary[2] #=> "abc"                   #Sugared equivalent
ary[0,2] #=> [1,2]  
ary[3][2][1] #=> 26  
[1,2,3][0] #=> 1  
"Is a string"[7,3] #=> "rin"  

Element assignment via instance methods Array#[]=, Hash#[]=, String#[]=, Struct#[]=, Thread#[]=, and class method ENV::[]=

ary = [1,2,3]  
ary.[]=(1,"abc") #=> [1,"abc",3]    #un-sugared notation
ary[2] = "def" #=> [1,"abc","def"]  #Sugared equivalent
hash = {"a"=>1, "b"=>2}  
hash["a"] = 3 #=> {"a"=>3, "b"=>2}  

Semantic constructs

Object creation via the array literal constructor

ary = []  

There are a bunch of literal constructors in Ruby that create an object of the relevant class via the use of (usually) a simple symbol pair, square brackets being the literal constructor for array objects: Array [], Hash {}, Proc ->(){}, Range .. and ..., Regexp //, String "" and '', Symbol : and :"".

Object creation via the % notation

%q[hello there you] #=> "hello there you"           # String % notation  
%w[hello there you] #=> ["hello", "there", "you"]   # Array % notation  

It is not, strictly speaking, square-bracket notation, but rather two-symbol-pair notation of which you can use square brackets if you wish. So %q@hello there you@ is equally valid.

Ruby’s regular expressions

/[^A-Fa-f0-9]/  

Square brackets indicate character classes in Ruby regular expressions.

I did find another use of the [], as a pattern for use in the Dir::glob method, but its supposed to act exactly as it does in regular expressions. Still, it indicates that there are possibly more uses hidden away in Ruby’s 1500+ methods.