diff --git a/mpact/sim/decoder/instruction_set.cc b/mpact/sim/decoder/instruction_set.cc
index 09ea0af..287f9b8 100644
--- a/mpact/sim/decoder/instruction_set.cc
+++ b/mpact/sim/decoder/instruction_set.cc
@@ -987,6 +987,19 @@
   absl::StrAppend(&array, "};\n\n");
   absl::StrAppend(&cc_output, array, "\n}  // namespace\n\n");
 
+  // Generate the regex matcher base class.
+  absl::StrAppend(
+      &h_output,
+      "// Assembly matcher.\n"
+      "class SlotMatcherInterface {\n"
+      " public:\n"
+      "  virtual ~SlotMatcherInterface() = default;\n"
+      "  virtual absl::StatusOr<std::tuple<uint64_t, int>> Encode(\n"
+      "      uint64_t address, absl::string_view text, int entry,\n"
+      "      ResolverInterface *resolver,\n"
+      "      std::vector<RelocationInfo> &relocations) = 0;\n"
+      "};\n\n");
+
   // Generate the regex matchers for each slot.
   for (auto *slot : slot_order_) {
     if (!slot->is_referenced()) continue;
diff --git a/mpact/sim/decoder/slot.cc b/mpact/sim/decoder/slot.cc
index 0918d6e..4997503 100644
--- a/mpact/sim/decoder/slot.cc
+++ b/mpact/sim/decoder/slot.cc
@@ -479,7 +479,7 @@
       &h_output,
       "// Assembly matcher.\n"
       "class ",
-      class_name,
+      class_name, " : public SlotMatcherInterface",
       " {\n"
       " public:\n"
       "  ",
@@ -492,7 +492,7 @@
       "absl::StatusOr<std::tuple<uint64_t, int>> "
       "  Encode(uint64_t address, absl::string_view text, int entry, "
       "ResolverInterface *resolver, std::vector<RelocationInfo> "
-      "&relocations);\n\n"
+      "&relocations) override;\n\n"
       " private:\n"
       "  bool Match(absl::string_view text, std::vector<int> &matches);\n"
       "  bool Extract(absl::string_view text, int index, "