QSyntaxHighlighter problem with highlighting opening and closing braces

Hi,
I am trying to make highlighting the opening and closing braces in formula editor.
suppose for example.
((a*b) +(a*c))
if i select the first opening braces it should highlight the corresponding(last one ) closing braces.
my problem is its highlighting for the first line correctly but for the second line if i am selecting its also showing for the previous line.
i am attaching the screen shot of output as well as my code.
i tried may thing but does not work.
please advise something will be appreciated.

thanks