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);

@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