<?php


    $subject = 'Jimmy turns 5+5 on Friday, Martha 2*2 on Saturday and Petra is 2*2 today.';
    
    $pattern = '~(\d+[*/+-]\d+)~e';
    
    echo preg_replace($pattern, '$1', $subject);

Jimmy turns 10 on Friday, Martha 4 on Saturday and Petra is 4 today.