nodejs request.url.match(regexp) seems to return a substring of the match in addition to the match

Question

nodejs request.url.match(regexp) seems to return a substring of the match in addition to the match

I tried a piece of code like this

var match = req.url.match(/^\/user\/(.+)/)

And passed a url like so "___http://someurl/user/jane"

as it turned out match was initialized to an array with the following elements

match[0]='/user/jane'
match[1]='jane'

I would have expected a single element i.e. the first element in match[0]. why was the second string returned -it doesn't seem to match the regex.

My experience with JavaScript is minimal and I couldn't find an explanation after some looking around. Appreciate an explanation of this

thanks

javascript
regex
node.js

Answer 1

Take a look at String.match, or better, RegExp.exec which has the same return value as String.match for a regex without the g flag:

The returned array has the matched text as the first item, and then one item for each capturing parenthesis that matched containing the text that was captured.

That is, a group between round brackets¹ makes a capturing group. If you only need the full match you can use:

var match = req.url.match(/^\/user\/.+/)[0];
console.log(match); //logs: "/user/jane"

This will extract the whole match (at index 0) returning it to the variable match, the rest of the array is discarded.

Note: If the regex may not match, you should test to see if it returns a match before extracting the full match to prevent against errors:

var match = req.url.match(/^\/user\/.+/);
if (match !== null) {
    match = match[0];
    console.log(match); //"/user/jane"
} else {
    console.log("no match");
}

Here's a live demo for fiddling around: jsFiddle

I've removed the capturing group as it wouldn't make a difference in this case. Doesn't actually matter, just a micro-optimization.

You can read more about Regular Expressions' capturing groups/backreferences here.

¹ Is not always the case, there are modifiers that make it a non-capturing group (?:), lookaheads, lookbehinds etc. but these are off-topic. You can find more about these in the site linked above.