diff --git a/lib/common/tests/rules/Makefile.am b/lib/common/tests/rules/Makefile.am
index 3343e2c8a6..7e8c38c391 100644
--- a/lib/common/tests/rules/Makefile.am
+++ b/lib/common/tests/rules/Makefile.am
@@ -1,18 +1,19 @@
 #
 # Copyright 2020-2024 the Pacemaker project contributors
 #
 # The version control history for this file may have further details.
 #
 # This source code is licensed under the GNU General Public License version 2
 # or later (GPLv2+) WITHOUT ANY WARRANTY.
 #
 
 include $(top_srcdir)/mk/tap.mk
 include $(top_srcdir)/mk/unittest.mk
 
 # Add "_test" to the end of all test program names to simplify .gitignore.
 check_PROGRAMS = pcmk__evaluate_date_expression_test	\
 		 pcmk__evaluate_date_spec_test		\
+		 pcmk__replace_submatches_test		\
 		 pcmk__unpack_duration_test
 
 TESTS = $(check_PROGRAMS)
diff --git a/lib/common/tests/rules/pcmk__replace_submatches_test.c b/lib/common/tests/rules/pcmk__replace_submatches_test.c
new file mode 100644
index 0000000000..e95f20c3e7
--- /dev/null
+++ b/lib/common/tests/rules/pcmk__replace_submatches_test.c
@@ -0,0 +1,79 @@
+/*
+ * Copyright 2024 the Pacemaker project contributors
+ *
+ * The version control history for this file may have further details.
+ *
+ * This source code is licensed under the GNU General Public License version 2
+ * or later (GPLv2+) WITHOUT ANY WARRANTY.
+ */
+
+#include <crm_internal.h>
+
+#include <regex.h>  // regmatch_t
+
+#include <crm/common/rules_internal.h>
+#include <crm/common/unittest_internal.h>
+
+// An example matched string with submatches
+static const char *match = "this is a string";
+static const regmatch_t submatches[] = {
+    { .rm_so = 0, .rm_eo = 16 },    // %0 = entire string
+    { .rm_so = 5, .rm_eo = 7 },     // %1 = "is"
+    { .rm_so = 9, .rm_eo = 9 },     // %2 = empty match
+};
+static const int nmatches = 3;
+
+static void
+assert_submatch(const char *string, const char *reference)
+{
+    char *expanded = NULL;
+
+    expanded = pcmk__replace_submatches(string, match, submatches, nmatches);
+    if ((expanded == NULL) || (reference == NULL)) {
+        assert_null(expanded);
+        assert_null(reference);
+    } else {
+        assert_int_equal(strcmp(expanded, reference), 0);
+    }
+    free(expanded);
+}
+
+static void
+no_source(void **state)
+{
+    assert_null(pcmk__replace_submatches(NULL, NULL, NULL, 0));
+    assert_submatch(NULL, NULL);
+    assert_submatch("", NULL);
+}
+
+static void
+source_has_no_variables(void **state)
+{
+    assert_submatch("this has no submatch variables",
+                    "this has no submatch variables");
+    assert_submatch("this ends in a %", "this ends in a %");
+    assert_submatch("%this starts with one", "%this starts with one");
+}
+
+static void
+without_matches(void **state)
+{
+    assert_submatch("this has an empty submatch %2",
+                    "this has an empty submatch ");
+    assert_submatch("this has a nonexistent submatch %3",
+                    "this has a nonexistent submatch ");
+}
+
+static void
+with_matches(void **state)
+{
+    assert_submatch("%0", match); // %0 matches entire string
+    assert_submatch("this %1", "this is");
+    assert_submatch("%1 this %ok", "is this %ok");
+}
+
+PCMK__UNIT_TEST(NULL, NULL,
+                cmocka_unit_test(no_source),
+                cmocka_unit_test(source_has_no_variables),
+                cmocka_unit_test(without_matches),
+                cmocka_unit_test(with_matches))