This is my patterns :
"X+X" => Example : 10+9
"X+X+X" => Example : 8+10+9
"X+(X+X)+X" => Example : 10+(8+6)+11
This is the rules :
The last number should be always "encapsulated" between italic marker like this : "10+<i>9</i>" , "8+10+<i>9</i>"
If two "+" symbols are present, the content between two "+" should be in bold like this : "8+<b>10</b>+<i>9</i>", "10+(<b>8+6</b>)+<i>11</i>
Have you an idea ?
Try this one
echo preg_replace(array('/\((.*)\)/','/\+([0-9])+\+/','/[0-9]+$/'), array('(<b>$1</b>)','+<b>$1</b>+','<i>$0</i>'), '11+5+3');