How to fetch username out of Email address

Hi fellow devs,

There is a scenario where I have email address available to me like: "testuser@gmail.com" and this email address has some username at the backend like this: "Test_user".

Now what I want to do is on the basis of email I want to fetch username. Previously I was using this function to fetch username out of email address:

index(getusersby(pv!ccperson,3,1),1, null)

(this function gives output in expression rule but in process model don't know why its not working and gives error)

but now it seems like this function is no longer working. I am unable to find any other function through which I can perform this functionality, any help in this regard would be helpful.

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