Hi @Marion,
Using a standard field format, this is not currently possible.
Effectively, the field format requires a 6-digit value, which means ZMB_Mam_###### will be the format it validates against. It requires the padded leading zeros to consider it a valid format.
You could alternatively use a regular expression as a field format instead by following this guide which would allow you to add both ZMB_Mam_44 and ZMB_Mam_135500 as values, but in that case, auto-incrementation would not be available.
The expression would look something like this:
^ZMB_Mam_\d{1,6}$
^indicates the start of stringZMB_Mam_this is the static prefix\d{1,6}allows for between 1 and 6 digits$terminates the string
You can use the “Alternate Catalog Number” field (altCatalogNumber) in the Collection Object table to capture the original number so that historical numbers are still available but do not prevent older records from being saved as each time a record is modified, it re-validates against the current format.