The document provides guidelines for creating a JavaScript regular expression (regex) to match valid usernames, which must consist of lowercase letters, numbers, underscores, or hyphens, and length between 3 to 16 characters. It includes examples of strings that comply or do not comply with these rules, emphasizing character limits. Additionally, it mentions regex for matching other data types such as passwords, hex values, slugs, emails, URLs, IP addresses, and HTML tags.