A method to generate ignored indexes and do partial replacement in a regex

regex-ignored-indexes.js

// Given an input
const input = "header 123 {#custom-id}"
// Provide a tranformation of said input that keeps the same length
// IE: "Capitalizing" a title in a markdown file
const transformedInput = input.toUpperCase();
// However, we don't want to transform this regex
// IE: A custom ID
const ignored = ["{#custom-id}"]

// From this, our output should be:
// "HEADER 123 {#custom-id}"

// Below is the implementation
let ignoredIndexes = new Set();
for (let regex of ignored) {
    const matches = input.match(new RegExp(regex, 'gmu'));
    if (!matches) continue;
    let lastIndex = 0;
    for (const match of matches) {
        const matchedIndex = input.indexOf(match, lastIndex);
        lastIndex = matchedIndex + match.length;
        for (let indexOfMatchLength = 0; indexOfMatchLength < match.length; indexOfMatchLength++) {
            ignoredIndexes.add(matchedIndex + indexOfMatchLength);
        }
    }
}

let output = "";
for (let inputIndex = 0; inputIndex < input.length; inputIndex++) {
    if (ignoredIndexes.has(inputIndex)) {
        output += input[inputIndex];
    } else {
        output += transformedInput[inputIndex];
    }
}

// Finally, output the result:
console.log(output);

I plan on contributing this to:

https://github.com/Xunnamius/unified-utils/tree/main/packages/remark-capitalize-headings

As a feature that suits my needs.

This is awesome @crutchcorn, why Regex though? It kinda looks like there could have been another way or maybe it's just cos I'm scared of Regex.

@tobySolutions good question! There's a few reasons:

1. The package in question already uses regexes extensively for this style of customization
2. Regexes can be easily serializable into JSON files, which is where many Remark config files live
3. While the demo uses a hardcoded custom ID, I need to be able to catch all custom IDs in my headers via this regex: /\{\s*#.*?\}\s*$/

Regexes aren't all that scary once you get used to them :) I even wrote a guide to them here:

https://unicorn-utterances.com/posts/the-complete-guide-to-regular-expressions-regex

Also, sorry for the disturbance, chatGPT also pointed out that there might be an issue with how the ignored characters was specified in the code:

https://chat.openai.com/share/fe811ff8-199d-48b1-b064-45144ca4151c

@tobySolutions That's intentional behavior😊

We don't want to escape the regexes, we want to capture them as-written

Thank you very much!! @crutchcorn