Strange string split behavior in Javascript

Question

When I run

'{{123}}'.split(/(\{\{)|(\}\})/g, -1)

I expect

["", "123", ""]

But it shows

["", "{{", undefined, "123", undefined, "}}", ""]

Why?? Is there anything wrong?

• David Thomas: Why have you passed a negative limit?

Answer 1

Use:

'{{123}}'.split(/\{\{|\}\}/)

You don't need to catch group when using .split

• pimvdb: You don't need the global flag, and -1 as a limit is not very useful either I think.

Answer 2

http://es5.github.com/#x15.5.4.14

If separator is a regular expression that contains capturing parentheses, then each time separator is matched the results (including any undefined results) of the capturing parentheses are spliced into the output array. For example,

"A<B>bold</B>and<CODE>coded</CODE>".split(/<(\/)?([^<>]+)>/)

evaluates to the array

["A", undefined, "B", "bold", "/", "B", "and", undefined, "CODE", "coded", "/", "CODE", ""]

So, '{{123}}'.split(/(\{\{)|(\}\})/g, -1) returns

["",
"{{", undefined, // "{{" is matched, but "}}" isn't.
"123",
undefined, "}}", // "{{" isn't matched, but "}}" is matched. 
""]