diff --git a/400_xowa/src/gplx/xowa/xtns/scribunto/libs/Scrib_lib_ustring__gsub__tst.java b/400_xowa/src/gplx/xowa/xtns/scribunto/libs/Scrib_lib_ustring__gsub__tst.java
index 1b05c9ce0..38bcfa74c 100644
--- a/400_xowa/src/gplx/xowa/xtns/scribunto/libs/Scrib_lib_ustring__gsub__tst.java
+++ b/400_xowa/src/gplx/xowa/xtns/scribunto/libs/Scrib_lib_ustring__gsub__tst.java
@@ -91,6 +91,11 @@ public class Scrib_lib_ustring__gsub__tst {
 		Exec_gsub("aa"			, "(a)%1"				, 1, "%1z", "az;1");	// capture; handle %1+; PAGE:en.w:Wikipedia:Wikipedia_Signpost/Templates/Voter/testcases; DATE:2015-08-02
 		Exec_gsub("a\"b'c\"d"	, "([\"'])(.-)%1"		, 1, "%1z", "a\"zd;1");	// capture; http://www.lua.org/pil/20.3.html; {{#invoke:test|gsub_string|a"b'c"d|(["'])(.-)%1|%1z}}
 	}
+	@Test  public void Regx__capture_wo_group() {
+		Exec_gsub("Ab", "%u", 1, "<%0>", "<A>b;1");
+		Exec_gsub("Ab", "%u", 1, "<%1>", "<A>b;1"); // NOTE: %1 should be same as %0 if no matches; ISSUE#:393; DATE:2019-03-20
+		Exec_gsub_fail("Ab", "%u", 1, "<%2>", "invalid capture index %2 in replacement String");
+	}
 	@Test  public void Regx__frontier_pattern() {	// PURPOSE: handle frontier pattern; EX:"%f[%a]"; DATE:2015-07-21
 		Exec_gsub("a b c", "%f[%W]", 5, "()", "a() b() c();3");
 		Exec_gsub("abC DEF gHI JKm NOP", "%f[%a]%u+%f[%A]", Int_.Max_value, "()", "abC () gHI JKm ();2");	// based on http://lua-users.org/wiki/FrontierPattern
@@ -120,6 +125,15 @@ public class Scrib_lib_ustring__gsub__tst {
 	private void Exec_gsub(String text, Object regx, int limit, Object repl, String expd) {
 		fxt.Test__proc__kvps__flat(lib, Scrib_lib_ustring.Invk_gsub, Scrib_kv_utl_.base1_many_(text, regx, repl, limit), expd);
 	}
+	private void Exec_gsub_fail(String text, Object regx, int limit, Object repl, String expd) {
+		try {
+			fxt.Test__proc__kvps__flat(lib, Scrib_lib_ustring.Invk_gsub, Scrib_kv_utl_.base1_many_(text, regx, repl, limit), expd);
+		} catch (Exception e) {
+			Gftest.Eq__bool(true, String_.Has(Err_.Message_gplx_log(e), expd));
+			return;
+		}
+		throw Err_.new_wo_type("expected failure");
+	}
 }
 class Mock_proc__recursive extends Mock_proc_fxt {	private final    Mock_scrib_fxt fxt; private final    Scrib_lib lib; private final    Mock_proc__recursive inner;
 	private final    Bry_bfr bfr;
diff --git a/400_xowa/src/gplx/xowa/xtns/scribunto/libs/Scrib_lib_ustring_gsub_mgr.java b/400_xowa/src/gplx/xowa/xtns/scribunto/libs/Scrib_lib_ustring_gsub_mgr.java
index 5d304861d..567d2f602 100644
--- a/400_xowa/src/gplx/xowa/xtns/scribunto/libs/Scrib_lib_ustring_gsub_mgr.java
+++ b/400_xowa/src/gplx/xowa/xtns/scribunto/libs/Scrib_lib_ustring_gsub_mgr.java
@@ -134,18 +134,21 @@ class Scrib_lib_ustring_gsub_mgr {
 									case Byte_ascii.Num_0: case Byte_ascii.Num_1: case Byte_ascii.Num_2: case Byte_ascii.Num_3: case Byte_ascii.Num_4:
 									case Byte_ascii.Num_5: case Byte_ascii.Num_6: case Byte_ascii.Num_7: case Byte_ascii.Num_8: case Byte_ascii.Num_9:
 										int idx = b - Byte_ascii.Num_0;
-										if (idx == 0)	// NOTE: 0 means take result; REF.MW:if ($x === '0'); return $m[0]; PAGE:Wikipedia:Wikipedia_Signpost/Templates/Voter/testcases; DATE:2015-08-02
+										// REF.MW: https://github.com/wikimedia/mediawiki-extensions-Scribunto/blob/master/includes/engines/LuaCommon/UstringLibrary.php#L785-L796
+										// NOTE: 0 means take result; REF.MW:if ($x === '0'); return $m[0]; PAGE:Wikipedia:Wikipedia_Signpost/Templates/Voter/testcases; DATE:2015-08-02
+										if (idx == 0)
 											tmp_bfr.Add_str_u8(String_.Mid(text, match.Find_bgn(), match.Find_end()));											
-										else {			// NOTE: > 0 means get from groups if it exists; REF.MW:elseif (isset($m["m$x"])) return $m["m$x"]; PAGE:Wikipedia:Wikipedia_Signpost/Templates/Voter/testcases; DATE:2015-08-02
-											idx -= List_adp_.Base1;
-											if (idx < match.Groups().length) {	// retrieve numbered capture; TODO_OLD: support more than 9 captures
-												Regx_group grp = match.Groups()[idx];
-												tmp_bfr.Add_str_u8(String_.Mid(text, grp.Bgn(), grp.End()));	// NOTE: grp.Bgn() / .End() is for String pos (bry pos will fail for utf8 strings)
-											}
-											else {
-												tmp_bfr.Add_byte(Byte_ascii.Percent);
-												tmp_bfr.Add_byte(b);
-											}
+										// NOTE: > 0 means get from groups if it exists; REF.MW:elseif (isset($m["m$x"])) return $m["m$x"]; PAGE:Wikipedia:Wikipedia_Signpost/Templates/Voter/testcases; DATE:2015-08-02
+										else if (idx - 1 < match.Groups().length) {	// retrieve numbered capture; TODO_OLD: support more than 9 captures
+											Regx_group grp = match.Groups()[idx - 1];
+											tmp_bfr.Add_str_u8(String_.Mid(text, grp.Bgn(), grp.End()));	// NOTE: grp.Bgn() / .End() is for String pos (bry pos will fail for utf8 strings)
+										}
+										// NOTE: 1 per MW "Match undocumented Lua String.gsub behavior"; PAGE:en.d:Wiktionary:Scripts ISSUE#:393; DATE:2019-03-20
+										else if (idx == 1) {
+											tmp_bfr.Add_str_u8(String_.Mid(text, match.Find_bgn(), match.Find_end()));
+										}
+										else {
+											throw Err_.new_wo_type("invalid capture index %" + Char_.To_str(b) + " in replacement String");
 										}
 										break;
 									case Byte_ascii.Percent: