MySQL: Count number of occurrences of a substring

Suppose you have a tweets table like this:

  +----------+-----------------------------------------------------------------------------------+
  | tweet_id | content                                                                           |
  +----------+-----------------------------------------------------------------------------------+
  | 1        | Traveling, exploring, and living my best life @JaneSmith @SaraJohnson @LisaTaylor |
  |          | @MikeBrown #Foodie #Fitness #Learning                                             | 
  | 2        | Just had the best dinner with friends! #Foodie #Friends #Fun                      |
  | 4        | Working hard on my new project #Work #Goals #Productivity #Fun                    |
  +----------+-----------------------------------------------------------------------------------+

And you want to find the number of hashtags (#) and mentions (@) in the content column. You can do so like this:

SELECT 
  *,
  LENGTH(REGEXP_REPLACE(content, '[^@]', '')) AS num_mentions,
  LENGTH(REGEXP_REPLACE(content, '[^#]', '')) AS num_hashtags
FROM tweets
ORDER BY tweet_id