If it is already stored as character, do this:
zipcode = put(input(zipcode,best.),z6.);
else, you need to convert to character:
zipcode_char = put(zipcode,z6.);
or you simply assign it the format for display:
format zipcode z6.;
Converting to number and then using a format provides for the simplest code. But you might to have to solve other issues (blanks within zipcodes etc) first for this to work.
In this like in most other cases, testing beats speculating
(German: "Probieren geht über studieren", English: "The proof of the pudding is in the eating")
Whilst I agree with the above, if number convert is easiest, if the data contains non numerics that isn't going to work, so here is a text version:
data want; input zip $; if lengthn(zip) < 6 then zip=cats(repeat("0",5-lengthn(zip)),zip); datalines; 12345 123456 12AE4 1 ; run;
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