Regex Practice 8/8

Capstone: normalizing usernames

The last lesson validated one username. Real input arrives as a batch - a signup list, an import file, a CSV column - and the job is usually "keep the valid ones, and clean them up." Regex answers the first half; ordinary array methods answer the second.

array.filter(fn) keeps only the items where fn returns true; array.map(fn) transforms what's left. Chain them and a messy list becomes exactly the usernames you can actually store: valid, and consistently lowercased.

Your task: write normalizeUsernames(list), returning only the valid usernames from list (3-16 characters, starts with a letter, only letters, digits, or underscores after that - same shape as the last lesson), each converted to lowercase, in their original order.

You'll practice:

Reusing a validation regex inside .filter()
Chaining .filter() and .map() to clean up a list

// Write normalizeUsernames(list): keep only valid usernames (3-16 chars,
// starts with a letter, then letters/digits/underscores), lowercased.
function normalizeUsernames(list) {

}