/usr/share/ucto/e-mail.rule is in libucto2 0.9.6-1build2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1
E-MAIL=^([\p{L}\p{N}\._%+\-]+@[\p{L}\p{N}\.\-]+\.\p{L}{2,4})\P{L}?$