/usr/share/acl2-7.1/books/projects/regex/ is in acl2-books 7.1-1.
This file is owned by root:root, with mode 0o755.
..
/usr/share/acl2-7.1/books/projects/regex/defset-encapsulates.o
/usr/share/acl2-7.1/books/projects/regex/defset-macros.o
/usr/share/acl2-7.1/books/projects/regex/equal-based-set.o
/usr/share/acl2-7.1/books/projects/regex/grep-command-line.o
/usr/share/acl2-7.1/books/projects/regex/input-list.o
/usr/share/acl2-7.1/books/projects/regex/portcullis.o
/usr/share/acl2-7.1/books/projects/regex/regex-chartrans.o
/usr/share/acl2-7.1/books/projects/regex/regex-defs.o
/usr/share/acl2-7.1/books/projects/regex/regex-exec.o
/usr/share/acl2-7.1/books/projects/regex/regex-fileio.o
/usr/share/acl2-7.1/books/projects/regex/regex-parse-brace.o
/usr/share/acl2-7.1/books/projects/regex/regex-parse-bracket.o
/usr/share/acl2-7.1/books/projects/regex/regex-parse.o
/usr/share/acl2-7.1/books/projects/regex/regex-tests.o
/usr/share/acl2-7.1/books/projects/regex/regex-ui.o