1. add asserts

If no error occurs, when the top level call returns:

- the stack should be empty
- state->repeat should be NULL

This prevents the following commits to wreck the stack.

Author: wjssz

Misc/NEWS.d/next/Library/2019-03-14-09-08-25.bpo-35859.8lFdLe.rst

@@ -0,0 +1,2 @@
+re module, fix a few bugs about capturing group. In rare cases, capturing
+group gets an incorrect string. Patch by Ma Lin.

Modules/sre_lib.h

@@ -1310,8 +1310,15 @@ SRE(match)(SRE_STATE* state, SRE_CODE* pattern, int toplevel)
     ctx_pos = ctx->last_ctx_pos;
     jump = ctx->jump;
     DATA_POP_DISCARD(ctx);
-    if (ctx_pos == -1)
+    if (ctx_pos == -1) {
+#ifdef Py_DEBUG
+        if (ctx->toplevel && ret >= 0) {
+            assert(state->repeat == NULL);
+            assert(state->data_stack_base == 0);
+        }
+#endif
         return ret;
+    }
     DATA_LOOKUP_AT(SRE(match_context), ctx, ctx_pos);
 
     switch (jump) {
@@ -1360,7 +1367,8 @@ SRE(match)(SRE_STATE* state, SRE_CODE* pattern, int toplevel)
             break;
     }
 
-    return ret; /* should never get here */
+    /* should never get here */
+    Py_UNREACHABLE();
 }
 
 /* need to reset capturing groups between two SRE(match) callings in loops */