Add regex for user fields

For ex: user email should end with vitstudent.ac.in
Reg No should be in the form like 25BCE0001

The issue names no files, tests, or entry points, so first locate where user email and registration-number fields are validated. Done means enforcing the stated email domain and registration-number format with regex checks, and confirming valid and invalid examples behave as expected.

Scritto dal modello di indicizzazione a partire dal testo della issue.