Hello. I'm trying to simply change PRItalicFormat markup to '~~' but it actually doesn't work with ~~ .

Also from the begining I had  ''this is italic''  test string on the Page. After I changed PRItalicFormats' markup from '''''''' to '~~' and then pressed "edit" for this page - there is ~~this is italic~~ in the TextArea. Seems to be correct... But when I'm pressing save button - there is not italic-styled string, but plain ~~this is italic~~ :/ Why so? How can I chage this markup?