Using PL/pgSQL text argument in 'IN (INT,INT,...)' clause

Good morning,

I have a function argument blah of type text containing something like
33,44,55,66
. Can I cast it in some way to use it in an IN clause as integers like
UPDATE foo SET x = y WHERE id IN ( blah );
or need I revert to dynamic SQL (EXECUTE...) ?

Thanks, Axel
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chaos claudius