How to extract value from a url - node.js

Question

I have a few requests coming in which follow the pattern below
contacts/id/
contacts/x/id/name
contacts/x/y/id/address
contacts/z/address/
I want to extract the value which follows right after 'contacts'
In above cases,
1. id
2. x
3. x
4. z
Here is my regex
(?<=contacts)\/[^\/]+
https://regex101.com/r/ePmv5Y/1
But it is matching along with the trailing '/' for eg. /id, /x etc
How do I optimize to get rid of this trailing slash?

Answer 1

We can use match() here:
var urls = ["contacts/id/", "contacts/x/id/name", "contacts/x/y/id/address", "contacts/z/address/"];
for (var i=0; i < urls.length; ++i) {
var output = urls[i].match(/\bcontacts\/(.*?)\//)[1];
console.log(urls[i] + " => " + output);
}

Answer 2

I have a few requests coming in
If you mean http requests, then this is likely the pathname of the requested URL, and they'll start with a /. (This is the value of req.url in a Node.js server.)
To match on a URL pathname, you can use this expression: ^\/contacts\/([^/?]+). Here's a link to another regular expression builder that demonstrates it and includes an explanation for every character: https://regexr.com/6tugf
The [^/?] is a negated set that matches any token which is not a / or a ? and the + means that it matches 1 or more of those tokens. It's important to include the ? because otherwise it could match into the query string portion of the URL — for example, in this URL:
https://domain.tld/contacts/x/id/name?filter=recent # URL
/contacts/x/id/name?filter=recent # req.url in Node.js
/contacts/x/id/name # pathname
?filter=recent # query string
And here's a runnable code snippet demonstrating the same expression, using String.prototype.match():
const contactIdRegexp = /^\/contacts\/([^/?]+)/;
const inputs = [
'/contacts/id/', // id
'/contacts/x/id/name', // x
'/contacts/x/y/id/address', // x
'/contacts/z/address/', // z
'/contacts/x/id/name?filter=recent', // x
];
for (const str of inputs) {
const id = str.match(contactIdRegexp)?.[1];
console.log(id);
}

Answer 3

You can add the / inside the lookbehind:
(?<=contacts\/)[^\/]+
See a regex demo.

Answer 4

If you like to continue without regex, You can try below.
//get the URL object.
const url = new URL(`${req.protocol}://${req.get('host')}${req.originalUrl}`);
//extract the pathname and split using "/"
const pathName= url.pathname.split("/");
//get the required value using array index.
const val = pathName[2];