Here's the Python code to extract and return the valid email addresses as a comma-separated list: ```python import re text = """ Contact information: 1. John Doe - john.doe@example.com 2. Jane Smith - jane@smith-family.net 3. support@company.co.uk 4. info@subdomain.example.org 5. Invalid email: not.an.email.com 6. Another invalid: @missing-username.com """ # Regular expression to match an email address email_regex = r'[\w\.-]+@[\w\.-]+' # Find all matches in the text emails = re.findall(email_regex, text) # Return the emails as a comma-separated list result = ",".join(emails) print(result) ``` Output: ``` john.doe@example.com,jane@smith-family.net,support@company.co.uk,info@subdomain.example.org ``` Explanation: 1. We use the `re` module in Python to work with regular expressions. 2. We define the regular expression pattern `email_regex` to match email addresses. The pattern `[\w\.-]+@[\w\.-]+` matches a word character (letters, digits, or underscores), a dot, or a hyphen one or more times (`[\w\.-]+`), followed by an at symbol `@`, and then another sequence of the same characters. 3. We use `re.findall()` to find all occurrences of the email pattern in the given `text`. This returns a list of all the matched email addresses. 4. We use the `join()` method to join the list of emails into a single string, separated by commas (`",".join(emails)`). 5. Finally, we print the resulting string containing the valid email addresses as a comma-separated list. The regular expression pattern used here may not cover all possible valid email syntax according to the official specification, but it covers a wide range of commonly used email formats. If you need a more comprehensive email validation, you may need to use a more sophisticated approach, such as using a dedicated library or defining a more complex regular expression pattern.