john.doe@example.com,jane@smith-family.net,support@company.co.uk,info@subdomain.example.org Explanation: 1. john.doe@example.com - Valid email 2. jane@smith-family.net - Valid email 3. support@company.co.uk - Valid email 4. info@subdomain.example.org - Valid email 5. not.an.email.com - Invalid email (no username before "@") 6. @missing-username.com - Invalid email (no username before "@", and starting with "@") To extract the valid email addresses, you can use regular expressions (regex) to find patterns that match the structure of an email address. The pattern for a valid email address is `^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$`. This pattern will look for: 1. One or more letters, numbers, dots, underscores, percent signs, plus signs or hyphens before the "@" symbol (`^[a-zA-Z0-9._%+-]+@`). 2. One or more letters, numbers, dots or hyphens after the "@" symbol and before the final dot (`[a-zA-Z0-9.-]+`). 3. Two or more letters after the final dot (`[a-zA-Z]{2,}$`). Once you have found all valid email addresses using this pattern, you can concatenate them into a single comma-separated string without spaces.