Replace the string after second forward slash using a RegEx

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:

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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

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

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

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

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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?

You can use

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

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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.

To also replace the next URL subpart, just add \/[^\/]* to the regex pattern:

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let myUrl = '/this/is/some/url';
const str1 = 'content1';
const str2 = 'content2'
const rx = /\/((?:[^\/]*\/){2})[^\/]*\/[^\/]*/;

const modifiedUrl = myUrl.replace(rx, '$1' + str1 + '/' + str2);

console.log(modifiedUrl); // this/is/content1/content2

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The main idea stays the same: capture the text before the text you want to replace, then just match what you need to replace, and - in the replacement - use a backreference to the text captured, and append the text to replace with.

NOTE: If your replacement text contains a literal $ followed with a digit that you actually do not want to treat as a backreference, but as a literal text, you should add .replace(/\$/g, '$$$$') to each variable, str1 and str2 in the code above.

I would choose an approach which breaks the task into two

• where the 1st replacement uses a regex which exclusively targets a string value's leading slash ... /^\// ... and ...

• where the 2nd replacement uses a regex which targets exactly the 3rd path-segment or path-partial ... /(?<=^(?:[^/]+\/){2})[^/]+/. The direct replacement here gets achieved by utilizing a positive lookbehind, where the latter targets a sequence of two following path-segments of the form xxx/yyy/.

The advantage comes with being able of processing both path variants alike, the ones with leading slash and the ones without the latter.

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console.log(
  'this/is/some/url ...',

  'this/is/some/url'
    // see ... [https://regex101.com/r/jfcZBU/2]
    .replace(/^\//, '')
    // see ... [https://regex101.com/r/jfcZBU/1]
    .replace(/(?<=^(?:[^/]+\/){2})[^/]+/, 'a')
);
console.log(
  '/this/is/some/url ...',

  '/this/is/some/url'
    .replace(/^\//, '')
    .replace(/(?<=^(?:[^/]+\/){2})[^/]+/, 'a')
);

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In case of combining/merging both regex patterns, also in favor of not having to use a positive lookbehind (though every relevant browser meanwhile does support this feature), the regex then becomes more complex, therefore more difficult to read ... /^\/*((?:[^/]+\/){2})[^/]+/.

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const regXPathCapture =
  // see ... [https://regex101.com/r/jfcZBU/3]
  /^\/*((?:[^/]+\/){2})[^/]+/;

console.log(
  "'this/is/some/url'.replace(regXPathCapture, '$1a') ...",
  'this/is/some/url'.replace(regXPathCapture, '$1a')
);
console.log(
  "'/this/is/some/url'.replace(regXPathCapture, '$1a') ...",
  '/this/is/some/url'.replace(regXPathCapture, '$1a')
);

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