Jacob Abel [off-list ref] writes:

+test_wt_add_excl () {
+	local opts="$*" &&
+	test_expect_success "'worktree add' with '$opts' has mutually exclusive options" '
+		test_must_fail git worktree add $opts 2>actual &&
+		grep -P "fatal:( options)? .* cannot be used together" actual
+	'
+}

Of course, "grep -P" is non-portable and CI jobs are easily broken.
Isn't -E (ERE) sufficient here?