The external parameters in your regular expression act as a capture group. From the split documentation ( https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/split ):
If the delimiter is a regular expression that contains a parenthesis, then each time the delimiter is matched, the results (including any undefined results) of the sliding parentheses are spliced into the output array.
You did not specify exactly what you want to achieve with your regular expression, perhaps you want something like this:
var str = "ab((cd))ef"; var arr = str.split(/[\)\(]+/); console.log(arr); // ["ab", "cd", "ef"]
EDIT:
Each bracket matches the regular expression individually, so the array looks like this (one line in brackets corresponds to:
['ab', '('] // matched ( ['ab', '(', '', '('] // matched ( (between the last two matches is the empty string ['ab', '(', '', '(', 'cd', ')'] // matched ) ['ab', '(', '', '(', 'cd', ')', '', ')'] // matched ) ['ab', '(', '', '(', 'cd', ')', '', ')', 'ef'] // string end
EDIT2:
Required output: ["ab", "(", "(", "cd", ")", ")", "ef"]
I'm not sure you can do this with one split. The fastest and safest way to do this is to simply filter out blank lines. I doubt there is a single section solution for regex.
var str = "ab((cd))ef"; var arr = str.split(/([\)\(])/).filter(function(item) { return item !== '';}); console.log(arr);