Fixed escaping of \u in Elasticsearch Query String queries

tests/test-backend-es-qs.py

@@ -86,7 +86,6 @@ async def check_queries():
                 try:
                     detail_result = await esa.search(index=index, q=query)
                 except Exception as e:
-                    #error = e.info['error']['root_cause']
                     error = e.info
 
                 failed.append((rule, query, error))

tools/sigma/backends.py

@@ -304,7 +304,7 @@ class ElasticsearchQuerystringBackend(SingleTextQueryBackend):
     identifier = "es-qs"
     active = True
 
-    reEscape = re.compile("([+\\-=!(){}\\[\\]^\"~:/]|\\\\(!>[*?])|&&|\\|\\|)")
+    reEscape = re.compile("([+\\-=!(){}\\[\\]^\"~:/]|\\\\(!>[*?])|\\\\u|&&|\\|\\|)")
     reClear = re.compile("[<>]")
     andToken = " AND "
     orToken = " OR "