problem with regexp_matches in nested funcion

I get this error:

ERROR: functions and operators can take at most one set argument
SQL state: 0A000

When I run this:

select message as before,
       replace(message,
               array_to_string(regexp_matches(message,'\\$\\$.*?\\{tabular\\}.*?\\$\\$'), '#'),
               replace(array_to_string(regexp_matches(message,'\\$\\$.*?\\{tabular\\}.*?\\$\\$'), '#'),'tabular','array')) as later
from mdl_forum_posts
where message ~ '\\$\\$.*?\\{tabular\\}.*?\\$\\$';

I know the problem is in the second array_to_string. Some idea?