User

Can only contain letters in lowercase

function isValidUsername(username) {
  return /^[a-z]+$/.test(username);
}

---

Password

Can contain any character, but must contain at least one lowercase letter, one uppercase letter, and one number.

Example 1

function isValidPassword(password) {
  return /[a-z]/.test(password) &&
         /[A-Z]/.test(password) &&
         /[0-9]/.test(password);
}

Example 2 (recommended)

function isValidPassword(password) {
  return /^(?=.*\d)(?=.*[a-z])(?=.*[A-Z]).*$/.test(password);
}

---

Telephone

Telephone number must be in the format of (555) 555-5555

function isValidTelephone(telephone) {
  return /^\(\d{3}\)\s\d{3}\-\d{4}$/.test(telephone);
}

---

Email

Must be a valid email address

function isValidEmail(email) {
  return /^[^@]+@[^@.]+\.[a-z]+$/i.test(email);
}

My solution

function isValidEmail(email) {
  return /^[a-z0-9-_]+@[a-z0-9-_]+\.[a-z]{2,}(\.[a-z]{2,})*$/i.test(email);
}

---

Color

function isHexColor(hexcolor) {
  return /^#[0-9a-fA-F]{6}$/i.test(hexcolor);