improve username validation

Closes #101

Author: kentcdodds

app/utils/user-validation.ts

@@ -4,7 +4,10 @@ export const usernameSchema = z
 	.string()
 	.min(3, { message: 'Username is too short' })
 	.max(20, { message: 'Username is too long' })
-	.regex(/^[^@]*$/, 'Username can not contain "@"')
+	.regex(/^[a-zA-Z0-9_]+$/, {
+		message: 'Username can only include letters, numbers, and underscores',
+	})
+
 export const passwordSchema = z
 	.string()
 	.min(6, { message: 'Password is too short' })