extmod/modure: Allow \\ in re.sub replacements.

extmod/modure.c

@@ -343,6 +343,9 @@ STATIC mp_obj_t re_sub_helper(size_t n_args, const mp_obj_t *args) {
                         const char *end_match = match->caps[match_no * 2 + 1];
                         vstr_add_strn(&vstr_return, start_match, end_match - start_match);
                     }
+                } else if (*repl == '\\') {
+                    // Add the \ character
+                    vstr_add_byte(&vstr_return, *repl++);
                 }
             } else {
                 // Just add the current byte from the replacement string

tests/extmod/ure_sub.py

@@ -43,6 +43,9 @@ print(
     )
 )
 
+# \g immediately followed by another \g
+print(re.sub("(abc)", r"\g<1>\g<1>", "abc"))
+
 # no matches at all
 print(re.sub("a", "b", "c"))
 
@@ -69,3 +72,6 @@ try:
     re.sub(123, "a", "a")
 except TypeError:
     print("TypeError")
+
+# Include \ in the sub replacement
+print(re.sub("b", "\\\\b", "abc"))