expr: Test $ at the end of a subpattern

Author: frendsick

tests/by-util/test_expr.rs

@@ -298,6 +298,10 @@ fn test_regex() {
         .args(&["a", ":", "a$\\|b"])
         .succeeds()
         .stdout_only("1\n");
+    new_ucmd!()
+        .args(&["ab", ":", "a\\(b$\\)"])
+        .succeeds()
+        .stdout_only("b\n");
     new_ucmd!()
         .args(&["abc", ":", "^abc"])
         .succeeds()