Goal

Transforming guest usernames from Microsoft Entra ID (formerly Azure AD) so that they match Atlassian usernames.
A guest username in Azure looks like the below, and a transformation will basically restore the email address part of the guest username. 

Prerequisites

• an Microsoft Entra ID (formerly Azure AD) subscription

• A (evaluation) subscription for the SAML SSO app for Atlassian Data Center or Server applications

• Admin access to your Atlassian Data Center or Server product

For more information about the prerequisites listed above, access the following link:

https://wiki.resolution.de/doc/saml-sso/latest/all/setup-guides-for-saml-sso/azure-ad

Step-by-Step Guide

1. Go to the configuration page of SAML SSO for Atlassian Data Center or Server
   
   

2. Select your identity provider (.How to transform Azure AD guest usernames v4.0.x#Image 01)
   
   

3. Scroll down to the Attribute Mapping table
    

4. Edit the Name-ID - Username Mapping (#Image 02)
   
   

5. Pick the NameID and convert Azure guest-user-UPN Template (#Image 03)
   
   

6. This is going to add the following transformation into the Regular Expression configuration Option.
   
   Regular expression: (.*)_(.*)#EXT#.*
Replacement: $1@$2
   
   You can also manually open this configuration and test with the Regex Replacement Tester(#Image 04)
   
   

7. Apply the configuration and Save your configuration.
   
   

Image 01: Select the entry for the Azure AD identity provider

Image 02: Edit NameID - Username Mapping

Image 03: Choose the predefined Template

Image 04: Display what the predefined Template has changed