/usr/share/acl2-6.3/books/regex/ is in acl2-books-certs 6.3-5.
This file is owned by root:root, with mode 0o755.
..
/usr/share/acl2-6.3/books/regex/defset-encapsulates.cert
/usr/share/acl2-6.3/books/regex/defset-macros.cert
/usr/share/acl2-6.3/books/regex/equal-based-set.cert
/usr/share/acl2-6.3/books/regex/grep-command-line.cert
/usr/share/acl2-6.3/books/regex/input-list.cert
/usr/share/acl2-6.3/books/regex/portcullis.cert
/usr/share/acl2-6.3/books/regex/regex-chartrans.cert
/usr/share/acl2-6.3/books/regex/regex-defs.cert
/usr/share/acl2-6.3/books/regex/regex-exec.cert
/usr/share/acl2-6.3/books/regex/regex-fileio.cert
/usr/share/acl2-6.3/books/regex/regex-parse-brace.cert
/usr/share/acl2-6.3/books/regex/regex-parse-bracket.cert
/usr/share/acl2-6.3/books/regex/regex-parse.cert
/usr/share/acl2-6.3/books/regex/regex-tests.cert
/usr/share/acl2-6.3/books/regex/regex-ui.cert