No anchor support for xtrabackup tables-exclude regular expression?

I create table “student”, “_student”, “sduent_suffix” three tables. if I invoke xtrabackup with --tables-exclude=“student”, it will exclude all three tables – which is right. however, invoke it with xtrabackup with --tables-exclude="^_student$", if anchor is supported, it should ignore just “_student” table and the other two should be backed up. The results is. All three is backed up — looks to me the regular expression is treat “^” and “$” as literally string instead of regular expression anchros

find the issue and ways to make it work as I want