On Regex Library use pattern \+(?:[^+\\]|\\[^+])*\+
and for Teststring
GroupNo+ \+ +Name+ / +Ac
And click test pattern.
Or in PHP:
$pattern = '/\+(?:[^+\\\\]|\\\\[^+])*\+/';
$string = 'GroupNo+ \+ +Name+ / +Ac';
preg_match_all($pattern, $string, $matches);