Get max value from an comma separated string

Hi, I've a field that contain values-comma-separated
like
A) 1;2;3;;5  -- ;2;;4;5
but also
B) 12;34;18
how I can get the max value?
For A I tried:
SELECT max(array_upper(string_to_array(answer,';'),1))
FROM values;
and work fine, but for B case I don't find a solution
like
SELECT max(string_to_array(answer,';')) FROM values;

Any ideas?
Thanks for any hint

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