MySQL 5.7: REGEXP version of PostgreSQL REGEXP_REPLACE

I have a CASE STATEMENT in PostgreSQL which I need to convert into a MySQL statement.

       CASE
        WHEN mobilenumber ~'^09[0-9]'
        THEN regexp_replace(mobilenumber, '0', '+63')
        WHEN mobilenumber ~'^9[0-9]' AND LENGTH(mobilenumber) = 10
        THEN '+63' || mobilenumber
        ELSE mobilenumber
      END

I am using MySQL 5.7. When I run the statement below, I am getting syntax error.

CASE
   WHEN mobilenumber regexp ~'^09[0-9]'
   THEN REGEXP(mobilenumber, '0', '+63')
END

Any advice please?

-------Update---------

CASE
   WHEN mobilenumber regexp ~'^09[0-9]'
   THEN REPLACE(mobilenumber, '0', '+63')
END

The problem is I am REGEXP in my THEN instead of REPLACE. I also need to remove the ~ in the WHEN statement. When I do that, all 0s are replaced by +63. Is there a way to control this?