It is possible to combine them, but it will be fiddly and not very efficient because it requires in some way checking the entire text for the existence of the specified substring when performing every single match. Here is one approach that combines a conditional operator with a dynamic command:
st1 = 'this is } and this is }, and this is }';
st2 = 'this is } and this is } and this is }';
rgx = '\}(?(?@contains([$`,$&,$''],''},'')),)';
regexp(st1,rgx)
regexp(st2,rgx)
Broken down:
% '\}(?(?@contains([$`,$&,$''],''},'')),)';
% \} literal curly brace
% (?( ),) conditional match comma
% ?@contains( , ) dynamic check text content:
% [$`,$&,$''] entire input text
% ''},'' literal curly-brace & comma
You could probably do something similar with lookarounds. But as noted, the condition command will be called again and again with each match... this is rather inefficient, because your condition does not change. I do not see an obvious way to avoid this. A more efficient approach (if you are able to relax your requirements) is probably to check that condition once before calling regexp:
rgx = '\}(?(?@tmp),)';
tmp = contains(st1,'},');
regexp(st1,rgx)
tmp = contains(st2,'},');
regexp(st2,rgx)
