[Accepted] MySQL
```
select
    content_id,
    content_text original_text,
    substr(
        regexp_replace(doubled_content_text, regex, "$1"),
        1,
        char_length(content_text)
    ) converted_text
from (
    select
        content_id,
        content_text,
        concat(lcase(content_text), ucase(content_text)) doubled_content_text,
        concat("(?<=[ -]|^)[a-z](?=.{", char_length(content_text) - 1, "}([a-z]))") regex
    from user_content
) temp
```
['MySQL']