Can I do this?

I am trying to create a function to automatically create a reference value
when a record is inserted into a table. I want the reference value to
consist of the user that is doing the insert, plus a couple of dates, plus
a sequence number, where the sequence number will increment every time a
given user inserts a record. because this sequence number is user specific,
my first thought is not to use a set of sequences for it, but to do this by
selecting the maximum sequence number that user has entered in the past.

So, I have a function that gets all the data, and concatenates it into a
string with the exception of the sequence. For that component, I have the
following test select that works.

SELECT NULLIF(regexp_replace(report_no, '\D','','g'), '')::numeric AS result
FROM
expense_report_instance
WHERE
/* NEW.project_key */ 123 = project_key;

But, when I add the requisite MAX clause, I get a syntax error.

SELECT MAX(NULLIF(regexp_replace(report_no, '\D','','g'), '')::numeric
FROM
expense_report_instance
WHERE
/* NEW.project_key */ 123 = project_key);

Is there a way I can make this work?

--
"They that would give up essential liberty for temporary safety deserve
neither liberty nor safety."
-- Benjamin Franklin