That sounds like an unusual limitation. Elasticsearch autogenerated IDs uses a form of base64 encoding that includes the characters - and _, as they are required in addition to the standard alphanumeric characters in order to get to 64 characters. You can not change this, which means that you may need to generate your own IDs.
Using fingerprint option to generate something else puts too much of the computational overhead.
Side note.
The application that poses limitation uses SOLR.
I wonder now if that limitation is because of its index digestion capability. But ES is based on Lucene, the same as SOLR.
Have you tried generating UUID through the fingerprint filter plugin? This should be computationally less expensive than the cryptographic hashes and would meet your format requirements.
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