/usr/share/acl2-8.0dfsg/books/projects/regex/ is in acl2-books-source 8.0dfsg-1.
This file is owned by root:root, with mode 0o755.
..
/usr/share/acl2-8.0dfsg/books/projects/regex/cert.acl2
/usr/share/acl2-8.0dfsg/books/projects/regex/defset-encapsulates.lisp
/usr/share/acl2-8.0dfsg/books/projects/regex/defset-macros.lisp
/usr/share/acl2-8.0dfsg/books/projects/regex/equal-based-set.lisp
/usr/share/acl2-8.0dfsg/books/projects/regex/grep-command-line.lisp
/usr/share/acl2-8.0dfsg/books/projects/regex/input-list.lisp
/usr/share/acl2-8.0dfsg/books/projects/regex/portcullis.acl2
/usr/share/acl2-8.0dfsg/books/projects/regex/portcullis.lisp
/usr/share/acl2-8.0dfsg/books/projects/regex/regex-chartrans.lisp
/usr/share/acl2-8.0dfsg/books/projects/regex/regex-defs.lisp
/usr/share/acl2-8.0dfsg/books/projects/regex/regex-exec.lisp
/usr/share/acl2-8.0dfsg/books/projects/regex/regex-fileio.lisp
/usr/share/acl2-8.0dfsg/books/projects/regex/regex-parse-brace.lisp
/usr/share/acl2-8.0dfsg/books/projects/regex/regex-parse-bracket.lisp
/usr/share/acl2-8.0dfsg/books/projects/regex/regex-parse.lisp
/usr/share/acl2-8.0dfsg/books/projects/regex/regex-tests.lisp
/usr/share/acl2-8.0dfsg/books/projects/regex/regex-ui.lisp