Example:
find and replace <script> bad stuff here </script> within a substring of a database table field. This snippit will replace the start and end tags for <script> and its contents.
---------------------------------------------------------------------------------
UPDATE table_name,
(
SELECT @start := '<script>', @end := '</script>'
)init
SET field_name= INSERT( field_name, LOCATE( @start , field_name) , LOCATE( @end , field_name) + CHAR_LENGTH( @end ) - LOCATE( @start , field_name) , '' )