Replace the string after second forward slash using a RegEx

July 12, 2023

Having a string (URL path) as for example: /this/is/some/url I would like to remove the first occurrence of / and change the string after the second forward slash (in the example some) by another string let’s say is. The result should be this/is/a/url and this is what I have:

let myUrl = '/this/is/some/url';
let splitUrl = myUrl.split('/').filter(v => v !== '');

splitUrl[2] = 'a';
let newUrl = splitUrl.join('/');

console.log(newUrl); // this/is/a/url

But now I wanted to change that into a RegEx so I came up with this:

const myUrl = '/this/is/some/url';
const modifiedUrl = myUrl.replace(/[a-zA-Z0-9]/, 'a').replace(/^\//, '');

console.log(modifiedUrl); // ahis/is/some/url

But the result is not what I want since it outputs: ahis/is/some/url. I’ll admit that I am not so good with RegEx.

A few SO posts here and here did not help me since I still did not get how to replace only the content after the second forward slash.

Can I get some help?

>Solution :

You can use

let myUrl = '/this/is/some/url';
const modifiedUrl = myUrl.replace(/\/((?:[^\/]*\/){2})[^\/]*/, '$1a');
console.log(modifiedUrl); // this/is/a/url

See this regex demo. Details:

\/ – a / char
((?:[^\/]*\/){2}) – Group 1: two occurrences of any zero or more chars other than / + a / char
[^\/]* – zero or more chars other than /.

The replacement is the backreference to the Group 1 value + the new string that will be inserted in the resulting string.