Username policies balance user freedom with community safety. Good implementation catches problematic usernames without frustrating legitimate users.
When to Check
Check usernames during registration before account creation. Also check during username changes. Provide real-time feedback as users type (debounced to avoid excessive API calls). This catches problems early rather than requiring moderation after the fact.
User-Friendly Error Messages
Don't say "username contains profanity"—this reveals your detection methods and may embarrass users who didn't intend offense. Instead use neutral language: "This username is not available" or "Please choose a different username." The goal is blocking bad usernames, not shaming users.
Combining with Other Checks
Username validation typically includes: format checks (length, allowed characters), profanity detection, availability check (not already taken), and possibly reserved name checks (admin, support, official). Run checks in order from cheapest to most expensive to fail fast.
Handling Edge Cases
Some usernames are borderline—potentially offensive but also possibly innocent. For low-risk platforms, you might allow them. For children's apps, err on blocking. Consider offering username suggestions when rejections happen. Track rejection rates to tune detection sensitivity.