Need help with regex

[rahul7514]

Hi Team
I am trying to build a query which uses office 365 and crowdstrike together 

My event section looks like this 

events:

        $event.metadata.vendor_name = "Crowdstrike" nocase

        $event.metadata.product_name = "Falcon"

        $event.metadata.log_type = "CS_DETECTS"

        $user = $event.principal.user.user_display_name

        $email.metadata.product_name = "Office 365"

        $email.metadata.event_type = "EMAIL_TRANSACTION"

        $email.metadata.product_event_type = "TIMailData"

        $email.about.labels["DeliveryAction"] = "Delivered"

        $email.security_result.category = "MAIL_PHISHING"

        $email.network.email.to =

assistance i want here is in the email address  i wish to remove the domain part and just capture the user 
For example from this email id  abcd.xyz@google.com  i want to capture just abcd.xyz
Can some one help me with this