LibRegex: Ensure the GoBack operation decrements the code unit index

This was missed in commit 27d555bab0.

Tests/LibRegex/Regex.cpp

@@ -637,6 +637,8 @@ TEST_CASE(ECMA262_match)
         { "(a{4}){2}"sv, "aaaaaaaa"sv },
         { "(a{4}){2}"sv, "aaaaaabaa"sv, false },
         { "\\u{4}"sv, "uuuu" },
+        { "(?<=.{3})f"sv, "abcdef"sv, true, (ECMAScriptFlags)regex::AllFlags::Global },
+        { "(?<=.{3})f"sv, "abc😀ef"sv, true, (ECMAScriptFlags)regex::AllFlags::Global },
         // ECMA262, B.1.4. Regular Expression Pattern extensions for browsers
         { "{"sv, "{"sv, true, ECMAScriptFlags::BrowserExtended },
         { "\\5"sv, "\5"sv, true, ECMAScriptFlags::BrowserExtended },
@@ -694,6 +696,8 @@ TEST_CASE(ECMA262_unicode_match)
         { "\\u{1f600}"sv, "😀"sv, true, ECMAScriptFlags::Unicode },
         { "\\ud83d\\ud83d"sv, "\xed\xa0\xbd\xed\xa0\xbd"sv, true },
         { "\\ud83d\\ud83d"sv, "\xed\xa0\xbd\xed\xa0\xbd"sv, true, ECMAScriptFlags::Unicode },
+        { "(?<=.{3})f"sv, "abcdef"sv, true, ECMAScriptFlags::Unicode },
+        { "(?<=.{3})f"sv, "abc😀ef"sv, true, ECMAScriptFlags::Unicode },
     };
 
     for (auto& test : tests) {

Userland/Libraries/LibRegex/RegexByteCode.cpp

@@ -102,6 +102,17 @@ static void advance_string_position(MatchState& state, RegexStringView const& vi
     }
 }
 
+static void reverse_string_position(MatchState& state, RegexStringView const& view, size_t amount)
+{
+    VERIFY(state.string_position >= amount);
+    state.string_position -= amount;
+
+    if (view.unicode())
+        state.string_position_in_code_units = view.code_unit_offset_of(state.string_position);
+    else
+        state.string_position_in_code_units -= amount;
+}
+
 static void save_string_position(MatchInput const& input, MatchState const& state)
 {
     input.saved_positions.append(state.string_position);
@@ -226,12 +237,12 @@ ALWAYS_INLINE ExecutionResult OpCode_Restore::execute(MatchInput const& input, M
     return ExecutionResult::Continue;
 }
 
-ALWAYS_INLINE ExecutionResult OpCode_GoBack::execute(MatchInput const&, MatchState& state) const
+ALWAYS_INLINE ExecutionResult OpCode_GoBack::execute(MatchInput const& input, MatchState& state) const
 {
     if (count() > state.string_position)
         return ExecutionResult::Failed_ExecuteLowPrioForks;
 
-    state.string_position -= count();
+    reverse_string_position(state, input.view, count());
     return ExecutionResult::Continue;
 }
 

Userland/Libraries/LibRegex/RegexMatch.h

@@ -274,6 +274,22 @@ public:
             });
     }
 
+    size_t code_unit_offset_of(size_t code_point_index) const
+    {
+        return m_view.visit(
+            [&](StringView const& view) -> u32 {
+                Utf8View utf8_view { view };
+                return utf8_view.byte_offset_of(code_point_index);
+            },
+            [&](Utf32View const&) -> u32 { return code_point_index; },
+            [&](Utf16View const& view) -> u32 {
+                return view.code_unit_offset_of(code_point_index);
+            },
+            [&](Utf8View const& view) -> u32 {
+                return view.byte_offset_of(code_point_index);
+            });
+    }
+
     bool operator==(char const* cstring) const
     {
         return m_view.visit(