Regex Practice 6/8

Practical validation

Real validation is rarely one trick - it's the pieces from the last five lessons, stacked. A username shape like "3 to 16 characters, must start with a letter, everything after can be a letter, digit, or underscore" reads directly as a regex once you know the vocabulary: an anchor to demand the whole string, a class for the first character, \w for the rest, and a quantifier for the count.

^[a-zA-Z]\w{2,15}$ - start, one letter, then 2 to 15 more word characters, end. That's a minimum total length of 3 and a maximum of 16. Same idea as the guide's "perfect email regex is a trap" warning: aim for a shape that's good enough and readable, not one that tries to catch everything.

Your task: write isValidUsername(str), true if str is 3-16 characters, starts with a letter, and contains only letters, digits, or underscores after that.

You'll practice:

Combining a character class, a quantifier, and anchors in one pattern
Reading a multi-part regex left to right as a sentence

// Write isValidUsername(str): 3-16 chars, starts with a letter,
// then only letters/digits/underscores.
function isValidUsername(str) {

}