replace first occurence of substring in string mysql

replace-first.sql

SET @string = 'this is TEST string with AA replace and An Answer';
SELECT @string as actual_string,
CONCAT(
    REPLACE(
        LEFT(
            @string,
            INSTR(@string, 'TEST') + LENGTH('TEST') - 1
        ),
        'TEST',
        'WTF'
    ),
    SUBSTRING(
        @string,
        INSTR(@string, 'TEST') + LENGTH('TEST') 
    )
) as new_string

based on http://stackoverflow.com/a/12123560.

now it can correctly replace occurences longer then 1 char