Question

I have next JS-code:

var str = 'some string';
var rexp = /^([^#]+)/;
var matchArr = str.match(rexp);

matchArr then contains two items matchArr[0] = ‘some string’ and
matchArr[1] = ‘some string’; Whereas I’m expecting the array with only one item.

I can’t understand this behavior. When I remove parentheses, then matchArr contains only one match. Why this happens, does anybody can explain?

Answer 1

When you use capturing parentheses in your regular expression, it adds an item to the returned results array for the overall match and each set of capturing parentheses.

matchArr[0] is everything that was matched.

matchArr[1] is what matches in the first set of capturing parentheses

matchArr[2] is what matches in the second set of capturing parentheses
and so on

So, in your regex /^([^#]+)/, there is no difference between matchArr[0] and matchArr[1] because everything that matches is in the capturing parentheses.

If you did this:

var str = 'some string';
var rexp = /^some([^#]+)/;
var matchArr = str.match(rexp);

You would find that:

matchArr[0] == "some string";
matchArr[1] == " string";

because there are parts of the match that are not in the capturing parentheses.

Or if you did this:

var str = 'some string';
var rexp = /^(some)([^#]+)/;
var matchArr = str.match(rexp);

You would find that:

matchArr[0] == "some string";
matchArr[1] == "some"
matchArr[2] == " string";