Copy regex unnamed captures to cloned subrequests. Previously, unnamed regex captures matched in the parent request, were not available in a cloned subrequest. Now 3 fields related to unnamed captures are copied to a cloned subrequest: r->ncaptures, r->captures and r->captures_data. Since r->captures cannot be changed by either request after creating a clone, a new flag r->realloc_captures is introduced to force reallocation of r->captures. The issue was reported as a proxy_cache_background_update misbehavior in http://mailman.nginx.org/pipermail/nginx/2018-December/057251.html.

--- a/src/http/ngx_http_core_module.c
+++ b/src/http/ngx_http_core_module.c
@@ -2386,6 +2386,14 @@ ngx_http_subrequest(ngx_http_request_t *
         sr->phase_handler = r->phase_handler;
         sr->write_event_handler = ngx_http_core_run_phases;
 
+#if (NGX_PCRE)
+        sr->ncaptures = r->ncaptures;
+        sr->captures = r->captures;
+        sr->captures_data = r->captures_data;
+        sr->realloc_captures = 1;
+        r->realloc_captures = 1;
+#endif
+
         ngx_http_update_location_config(sr);
     }