Regex_INSTR(Column,'[[:digit:]]') will return the position of the first digit in a character string so you could use it to determine, if your string (or a record field value passed as a parameter to the function) contains digits. Transferring this to your example, it would look like:
case when Regex_INSTR(Column,'[[:digit:]]') > 0
end column1 ,
case when Regex_INSTR(Column,'[[:digit:]]') <= 0
Note that it's not the best example you're giving, as there isn't any need to do double selections and you could simply use
CASE WHEN THEN ELSE END block instead.