How can I substitute a specific capture group instead of the entire match using a regular expression?

Question

How can I substitute a specific capture group instead of the entire match using a regular expression?

I'm struggling with the following code snippet:

let x = "the *quick* brown fox";

let y = x.replace(/[^\\](\*)(.*)(\*)/g, "<strong>$2</strong>");

console.log(y);

This piece of code replaces *quick* with

<strong>quick</strong>

The issue arises when I want to use backslashes as an escape character, so that \*quick* remains unchanged, instead of being converted to

<strong>quick</strong>

To address this, I have added a condition in the regex to check for the absence of a backslash before the asterisks.

However, this approach also removes that preceding character in the final output.

As a result, the text becomes:

the<strong>quick</strong> brown fox

(notice the missing space)

My thought was to encapsulate the latter part of the regex in a capturing group and replace only that specific segment. Yet, I believe Javascript's replace function operates solely on the entire match.

Is there a solution to overcome this hurdle? I am hesitant to opt for a comprehensive markdown library as I merely require handling a few scenarios like bolding and italics. Hence, my intention is to keep things straightforward.

javascript regex string markdown

Answer 1

Answer №1

Implement a negative lookbehind in order to exclude the character from the matched result. This adjustment will also enable it to match at the start of the string (since your regular expression currently requires a preceding character for a match).

let x = "the *quick* brown fox";
let y = x.replace(/(?<!\\)(\*)(.*)(\*)/g, "<strong>$2</strong>");
console.log(y);

x = "the \\*quick* brown fox";
y = x.replace(/(?<!\\)(\*)(.*)(\*)/g, "<strong>$2</strong>");
console.log(y);

Negative lookbehinds represent a new addition to JavaScript, so if you need to support older browsers, place the preceding character into another capture group and include it in the replacement. To address the issue of matching at the beginning, ensure that it matches either the start of the string or a non-backslash character.

let x = "the \\*quick* brown fox";
let y = x.replace(/(^|[^\\])(\*)(.*)(\*)/g, "$1<strong>$3</strong>");
console.log(y);

x = "the *quick* brown fox";
y = x.replace(/(^|[^\\])(\*)(.*)(\*)/g, "$1<strong>$3</strong>");
console.log(y);

Answer 2

Implement a negative lookbehind in order to exclude the character from the matched result. This adjustment will also enable it to match at the start of the string (since your regular expression currently requires a preceding character for a match).

let x = "the *quick* brown fox";
let y = x.replace(/(?<!\\)(\*)(.*)(\*)/g, "<strong>$2</strong>");
console.log(y);

x = "the \\*quick* brown fox";
y = x.replace(/(?<!\\)(\*)(.*)(\*)/g, "<strong>$2</strong>");
console.log(y);

Negative lookbehinds represent a new addition to JavaScript, so if you need to support older browsers, place the preceding character into another capture group and include it in the replacement. To address the issue of matching at the beginning, ensure that it matches either the start of the string or a non-backslash character.

let x = "the \\*quick* brown fox";
let y = x.replace(/(^|[^\\])(\*)(.*)(\*)/g, "$1<strong>$3</strong>");
console.log(y);

x = "the *quick* brown fox";
y = x.replace(/(^|[^\\])(\*)(.*)(\*)/g, "$1<strong>$3</strong>");
console.log(y);

How can I substitute a specific capture group instead of the entire match using a regular expression?

Answer №1

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