ok, thank you for your advice. I can't show the real data but, I have a column called user_id which contains values like: 123456789 12345678 9862416351 (which are the VAT numbers) in the same used_id column there are also other values like: lcnfno19f44h273g frcfvr77d31z976m (which instead is the tax code (in Italy)) both types of values are user identifiers. the first 3 lines of the example are 9,8,10 digits respectively, but should be 11, this is because when downloading the excel file from the portal from which I got the data, the USER_ID column was probably loaded as numerical and did not consider the first digits of the VAT number which are zero. so the column values I would like to get are: 00123456789 00012345678 09862416351 and leave the other lines referring to tax codes unchanged. to do this I saw that the zw.d format can be applied, which however is only applicable to numeric values. but I don't know how to select only the numeric values of the USER_ID column. I hope I was clearer.
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