To be more precise, uid is the field that contains the user identifier and it may contains -, as of today we are rewriting it on the IDP but we may have to change the IDP to another one that don't allow claims rewrite.
So we want to find a way to rewrite uid field to replace - by _.
The following isn't allowed on the mapping table :
Map from replaceAll(uid,"-","_") , Map to .pyUserIdentifier
By tracing the request we found that pyLoadSAMLAssertion is called to build D_SAMLAssertionDataPage but we don't know how we can overwrite it.