diff --git a/core/fxcrt/fx_basic_bstring.cpp b/core/fxcrt/fx_basic_bstring.cpp
index ab92996..10bc05b 100644
--- a/core/fxcrt/fx_basic_bstring.cpp
+++ b/core/fxcrt/fx_basic_bstring.cpp
@@ -420,27 +420,20 @@
   pOldData->Release();
 }
 CFX_ByteString CFX_ByteString::Mid(FX_STRSIZE nFirst) const {
-  if (!m_pData) {
+  if (!m_pData)
     return CFX_ByteString();
-  }
+
   return Mid(nFirst, m_pData->m_nDataLength - nFirst);
 }
 CFX_ByteString CFX_ByteString::Mid(FX_STRSIZE nFirst, FX_STRSIZE nCount) const {
-  if (nFirst < 0) {
-    nFirst = 0;
-  }
-  if (nCount < 0) {
-    nCount = 0;
-  }
-  if (nFirst + nCount > m_pData->m_nDataLength) {
-    nCount = m_pData->m_nDataLength - nFirst;
-  }
-  if (nFirst > m_pData->m_nDataLength) {
-    nCount = 0;
-  }
-  if (nFirst == 0 && nFirst + nCount == m_pData->m_nDataLength) {
+  if (!m_pData)
+    return CFX_ByteString();
+
+  nFirst = std::min(std::max(0, nFirst), m_pData->m_nDataLength);
+  nCount = std::min(std::max(0, nCount), m_pData->m_nDataLength - nFirst);
+  if (nFirst == 0 && nCount == m_pData->m_nDataLength)
     return *this;
-  }
+
   CFX_ByteString dest;
   AllocCopy(dest, nCount, nFirst);
   return dest;
diff --git a/core/fxcrt/fx_basic_bstring_unittest.cpp b/core/fxcrt/fx_basic_bstring_unittest.cpp
index 834a6e4..7dc4db5 100644
--- a/core/fxcrt/fx_basic_bstring_unittest.cpp
+++ b/core/fxcrt/fx_basic_bstring_unittest.cpp
@@ -315,6 +315,224 @@
   EXPECT_EQ("xxxxxx", not_aliased);
 }
 
+TEST(fxcrt, ByteStringRemove) {
+  CFX_ByteString freed("FREED");
+  freed.Remove('E');
+  EXPECT_EQ("FRD", freed);
+  freed.Remove('F');
+  EXPECT_EQ("RD", freed);
+  freed.Remove('D');
+  EXPECT_EQ("R", freed);
+  freed.Remove('X');
+  EXPECT_EQ("R", freed);
+  freed.Remove('R');
+  EXPECT_EQ("", freed);
+
+  CFX_ByteString empty;
+  empty.Remove('X');
+  EXPECT_EQ("", empty);
+}
+
+TEST(fxcrt, ByteStringReplace) {
+  CFX_ByteString fred("FRED");
+  fred.Replace("FR", "BL");
+  EXPECT_EQ("BLED", fred);
+  fred.Replace("D", "DDY");
+  EXPECT_EQ("BLEDDY", fred);
+  fred.Replace("LEDD", "");
+  EXPECT_EQ("BY", fred);
+  fred.Replace("X", "CLAMS");
+  EXPECT_EQ("BY", fred);
+  fred.Replace("BY", "HI");
+  EXPECT_EQ("HI", fred);
+  fred.Replace("", "CLAMS");
+  EXPECT_EQ("HI", fred);
+  fred.Replace("HI", "");
+  EXPECT_EQ("", fred);
+}
+
+TEST(fxcrt, ByteStringInsert) {
+  CFX_ByteString fred("FRED");
+  fred.Insert(-1, 'X');
+  EXPECT_EQ("XFRED", fred);
+  fred.Insert(0, 'S');
+  EXPECT_EQ("SXFRED", fred);
+  fred.Insert(2, 'T');
+  EXPECT_EQ("SXTFRED", fred);
+  fred.Insert(5, 'U');
+  EXPECT_EQ("SXTFRUED", fred);
+  fred.Insert(8, 'V');
+  EXPECT_EQ("SXTFRUEDV", fred);
+  fred.Insert(12, 'P');
+  EXPECT_EQ("SXTFRUEDVP", fred);
+  {
+    CFX_ByteString empty;
+    empty.Insert(-1, 'X');
+    EXPECT_EQ("X", empty);
+  }
+  {
+    CFX_ByteString empty;
+    empty.Insert(0, 'X');
+    EXPECT_EQ("X", empty);
+  }
+  {
+    CFX_ByteString empty;
+    empty.Insert(5, 'X');
+    EXPECT_EQ("X", empty);
+  }
+}
+
+TEST(fxcrt, ByteStringDelete) {
+  CFX_ByteString fred("FRED");
+  fred.Delete(0, 2);
+  EXPECT_EQ("ED", fred);
+  fred.Delete(1);
+  EXPECT_EQ("E", fred);
+  fred.Delete(-1);
+  EXPECT_EQ("", fred);
+  fred.Delete(1);
+  EXPECT_EQ("", fred);
+
+  CFX_ByteString empty;
+  empty.Delete(0);
+  EXPECT_EQ("", empty);
+  empty.Delete(-1);
+  EXPECT_EQ("", empty);
+  empty.Delete(1);
+  EXPECT_EQ("", empty);
+}
+
+TEST(fxcrt, ByteStringMid) {
+  CFX_ByteString fred("FRED");
+  EXPECT_EQ("", fred.Mid(0, 0));
+  EXPECT_EQ("", fred.Mid(3, 0));
+  EXPECT_EQ("FRED", fred.Mid(0));
+  EXPECT_EQ("RED", fred.Mid(1));
+  EXPECT_EQ("ED", fred.Mid(2));
+  EXPECT_EQ("D", fred.Mid(3));
+  EXPECT_EQ("F", fred.Mid(0, 1));
+  EXPECT_EQ("R", fred.Mid(1, 1));
+  EXPECT_EQ("E", fred.Mid(2, 1));
+  EXPECT_EQ("D", fred.Mid(3, 1));
+  EXPECT_EQ("FR", fred.Mid(0, 2));
+  EXPECT_EQ("FRED", fred.Mid(0, 4));
+  EXPECT_EQ("FRED", fred.Mid(0, 10));
+
+  EXPECT_EQ("FR", fred.Mid(-1, 2));
+  EXPECT_EQ("RED", fred.Mid(1, 4));
+  EXPECT_EQ("", fred.Mid(4, 1));
+
+  CFX_ByteString empty;
+  EXPECT_EQ("", empty.Mid(0, 0));
+  EXPECT_EQ("", empty.Mid(0));
+  EXPECT_EQ("", empty.Mid(1));
+  EXPECT_EQ("", empty.Mid(-1));
+}
+
+TEST(fxcrt, ByteStringLeft) {
+  CFX_ByteString fred("FRED");
+  EXPECT_EQ("", fred.Left(0));
+  EXPECT_EQ("F", fred.Left(1));
+  EXPECT_EQ("FR", fred.Left(2));
+  EXPECT_EQ("FRE", fred.Left(3));
+  EXPECT_EQ("FRED", fred.Left(4));
+
+  EXPECT_EQ("FRED", fred.Left(5));
+  EXPECT_EQ("", fred.Left(-1));
+
+  CFX_ByteString empty;
+  EXPECT_EQ("", empty.Left(0));
+  EXPECT_EQ("", empty.Left(1));
+  EXPECT_EQ("", empty.Left(-1));
+}
+
+TEST(fxcrt, ByteStringRight) {
+  CFX_ByteString fred("FRED");
+  EXPECT_EQ("", fred.Right(0));
+  EXPECT_EQ("D", fred.Right(1));
+  EXPECT_EQ("ED", fred.Right(2));
+  EXPECT_EQ("RED", fred.Right(3));
+  EXPECT_EQ("FRED", fred.Right(4));
+
+  EXPECT_EQ("FRED", fred.Right(5));
+  EXPECT_EQ("", fred.Right(-1));
+
+  CFX_ByteString empty;
+  EXPECT_EQ("", empty.Right(0));
+  EXPECT_EQ("", empty.Right(1));
+  EXPECT_EQ("", empty.Right(-1));
+}
+
+TEST(fxcrt, ByteStringUpperLower) {
+  CFX_ByteString fred("F-Re.42D");
+  fred.MakeLower();
+  EXPECT_EQ("f-re.42d", fred);
+  fred.MakeUpper();
+  EXPECT_EQ("F-RE.42D", fred);
+
+  CFX_ByteString empty;
+  empty.MakeLower();
+  EXPECT_EQ("", empty);
+  empty.MakeUpper();
+  EXPECT_EQ("", empty);
+}
+
+TEST(fxcrt, ByteStringTrimRight) {
+  CFX_ByteString fred("  FRED  ");
+  fred.TrimRight();
+  EXPECT_EQ("  FRED", fred);
+  fred.TrimRight('E');
+  EXPECT_EQ("  FRED", fred);
+  fred.TrimRight('D');
+  EXPECT_EQ("  FRE", fred);
+  fred.TrimRight("ERP");
+  EXPECT_EQ("  F", fred);
+
+  CFX_ByteString blank("   ");
+  blank.TrimRight("ERP");
+  EXPECT_EQ("   ", blank);
+  blank.TrimRight('E');
+  EXPECT_EQ("   ", blank);
+  blank.TrimRight();
+  EXPECT_EQ("", blank);
+
+  CFX_ByteString empty;
+  empty.TrimRight("ERP");
+  EXPECT_EQ("", empty);
+  empty.TrimRight('E');
+  EXPECT_EQ("", empty);
+  empty.TrimRight();
+  EXPECT_EQ("", empty);
+}
+
+TEST(fxcrt, ByteStringTrimLeft) {
+  CFX_ByteString fred("  FRED  ");
+  fred.TrimLeft();
+  EXPECT_EQ("FRED  ", fred);
+  fred.TrimLeft('E');
+  EXPECT_EQ("FRED  ", fred);
+  fred.TrimLeft('F');
+  EXPECT_EQ("RED  ", fred);
+  fred.TrimLeft("ERP");
+  EXPECT_EQ("D  ", fred);
+
+  CFX_ByteString blank("   ");
+  blank.TrimLeft("ERP");
+  EXPECT_EQ("   ", blank);
+  blank.TrimLeft('E');
+  EXPECT_EQ("   ", blank);
+  blank.TrimLeft();
+  EXPECT_EQ("", blank);
+
+  CFX_ByteString empty;
+  empty.TrimLeft("ERP");
+  EXPECT_EQ("", empty);
+  empty.TrimLeft('E');
+  EXPECT_EQ("", empty);
+  empty.TrimLeft();
+  EXPECT_EQ("", empty);
+}
+
 TEST(fxcrt, ByteStringCNotNull) {
   CFX_ByteStringC string3("abc");
   CFX_ByteStringC string6("abcdef");
diff --git a/core/fxcrt/fx_basic_wstring.cpp b/core/fxcrt/fx_basic_wstring.cpp
index aa62b5e..642b75e 100644
--- a/core/fxcrt/fx_basic_wstring.cpp
+++ b/core/fxcrt/fx_basic_wstring.cpp
@@ -441,27 +441,20 @@
   return dest;
 }
 CFX_WideString CFX_WideString::Mid(FX_STRSIZE nFirst) const {
+  if (!m_pData)
+    return CFX_WideString();
+
   return Mid(nFirst, m_pData->m_nDataLength - nFirst);
 }
 CFX_WideString CFX_WideString::Mid(FX_STRSIZE nFirst, FX_STRSIZE nCount) const {
-  if (!m_pData) {
+  if (!m_pData)
     return CFX_WideString();
-  }
-  if (nFirst < 0) {
-    nFirst = 0;
-  }
-  if (nCount < 0) {
-    nCount = 0;
-  }
-  if (nFirst + nCount > m_pData->m_nDataLength) {
-    nCount = m_pData->m_nDataLength - nFirst;
-  }
-  if (nFirst > m_pData->m_nDataLength) {
-    nCount = 0;
-  }
-  if (nFirst == 0 && nFirst + nCount == m_pData->m_nDataLength) {
+
+  nFirst = std::min(std::max(nFirst, 0), m_pData->m_nDataLength);
+  nCount = std::min(std::max(nCount, 0), m_pData->m_nDataLength - nFirst);
+  if (nFirst == 0 && nCount == m_pData->m_nDataLength)
     return *this;
-  }
+
   CFX_WideString dest;
   AllocCopy(dest, nCount, nFirst);
   return dest;
diff --git a/core/fxcrt/fx_basic_wstring_unittest.cpp b/core/fxcrt/fx_basic_wstring_unittest.cpp
index a64d7a9..8f00846 100644
--- a/core/fxcrt/fx_basic_wstring_unittest.cpp
+++ b/core/fxcrt/fx_basic_wstring_unittest.cpp
@@ -276,6 +276,230 @@
   EXPECT_EQ(L"xxxxxx", not_aliased);
 }
 
+TEST(fxcrt, WideStringRemove) {
+  CFX_WideString freed(L"FREED");
+  freed.Remove(L'E');
+  EXPECT_EQ(L"FRD", freed);
+  freed.Remove(L'F');
+  EXPECT_EQ(L"RD", freed);
+  freed.Remove(L'D');
+  EXPECT_EQ(L"R", freed);
+  freed.Remove(L'X');
+  EXPECT_EQ(L"R", freed);
+  freed.Remove(L'R');
+  EXPECT_EQ(L"", freed);
+
+  CFX_WideString empty;
+  empty.Remove(L'X');
+  EXPECT_EQ(L"", empty);
+}
+
+TEST(fxcrt, WideStringReplace) {
+  CFX_WideString fred(L"FRED");
+  fred.Replace(L"FR", L"BL");
+  EXPECT_EQ(L"BLED", fred);
+  fred.Replace(L"D", L"DDY");
+  EXPECT_EQ(L"BLEDDY", fred);
+  fred.Replace(L"LEDD", L"");
+  EXPECT_EQ(L"BY", fred);
+  fred.Replace(L"X", L"CLAMS");
+  EXPECT_EQ(L"BY", fred);
+  fred.Replace(L"BY", L"HI");
+  EXPECT_EQ(L"HI", fred);
+  fred.Replace(L"", L"CLAMS");
+  EXPECT_EQ(L"HI", fred);
+  fred.Replace(L"HI", L"");
+  EXPECT_EQ(L"", fred);
+}
+
+TEST(fxcrt, WideStringInsert) {
+  CFX_WideString fred(L"FRED");
+  fred.Insert(-1, 'X');
+  EXPECT_EQ(L"XFRED", fred);
+
+  fred.Insert(0, 'S');
+  EXPECT_EQ(L"SXFRED", fred);
+
+  fred.Insert(2, 'T');
+  EXPECT_EQ(L"SXTFRED", fred);
+
+  fred.Insert(5, 'U');
+  EXPECT_EQ(L"SXTFRUED", fred);
+
+  fred.Insert(8, 'V');
+  EXPECT_EQ(L"SXTFRUEDV", fred);
+
+  fred.Insert(12, 'P');
+  EXPECT_EQ(L"SXTFRUEDVP", fred);
+
+  {
+    CFX_WideString empty;
+    empty.Insert(-1, 'X');
+    EXPECT_EQ(L"X", empty);
+  }
+  {
+    CFX_WideString empty;
+    empty.Insert(0, 'X');
+    EXPECT_EQ(L"X", empty);
+  }
+  {
+    CFX_WideString empty;
+    empty.Insert(5, 'X');
+    EXPECT_EQ(L"X", empty);
+  }
+}
+
+TEST(fxcrt, WideStringDelete) {
+  CFX_WideString fred(L"FRED");
+  fred.Delete(0, 2);
+  EXPECT_EQ(L"ED", fred);
+  fred.Delete(1);
+  EXPECT_EQ(L"E", fred);
+  fred.Delete(-1);
+  EXPECT_EQ(L"", fred);
+  fred.Delete(1);
+  EXPECT_EQ(L"", fred);
+
+  CFX_WideString empty;
+  empty.Delete(0);
+  EXPECT_EQ(L"", empty);
+  empty.Delete(-1);
+  EXPECT_EQ(L"", empty);
+  empty.Delete(1);
+  EXPECT_EQ(L"", empty);
+}
+
+TEST(fxcrt, WideStringMid) {
+  CFX_WideString fred(L"FRED");
+  EXPECT_EQ(L"", fred.Mid(0, 0));
+  EXPECT_EQ(L"", fred.Mid(3, 0));
+  EXPECT_EQ(L"FRED", fred.Mid(0));
+  EXPECT_EQ(L"RED", fred.Mid(1));
+  EXPECT_EQ(L"ED", fred.Mid(2));
+  EXPECT_EQ(L"D", fred.Mid(3));
+  EXPECT_EQ(L"F", fred.Mid(0, 1));
+  EXPECT_EQ(L"R", fred.Mid(1, 1));
+  EXPECT_EQ(L"E", fred.Mid(2, 1));
+  EXPECT_EQ(L"D", fred.Mid(3, 1));
+  EXPECT_EQ(L"FR", fred.Mid(0, 2));
+  EXPECT_EQ(L"FRED", fred.Mid(0, 4));
+  EXPECT_EQ(L"FRED", fred.Mid(0, 10));
+
+  EXPECT_EQ(L"FR", fred.Mid(-1, 2));
+  EXPECT_EQ(L"RED", fred.Mid(1, 4));
+  EXPECT_EQ(L"", fred.Mid(4, 1));
+
+  CFX_WideString empty;
+  EXPECT_EQ(L"", empty.Mid(0, 0));
+  EXPECT_EQ(L"", empty.Mid(0));
+  EXPECT_EQ(L"", empty.Mid(1));
+  EXPECT_EQ(L"", empty.Mid(-1));
+}
+
+TEST(fxcrt, WideStringLeft) {
+  CFX_WideString fred(L"FRED");
+  EXPECT_EQ(L"", fred.Left(0));
+  EXPECT_EQ(L"F", fred.Left(1));
+  EXPECT_EQ(L"FR", fred.Left(2));
+  EXPECT_EQ(L"FRE", fred.Left(3));
+  EXPECT_EQ(L"FRED", fred.Left(4));
+
+  EXPECT_EQ(L"FRED", fred.Left(5));
+  EXPECT_EQ(L"", fred.Left(-1));
+
+  CFX_WideString empty;
+  EXPECT_EQ(L"", empty.Left(0));
+  EXPECT_EQ(L"", empty.Left(1));
+  EXPECT_EQ(L"", empty.Left(-1));
+}
+
+TEST(fxcrt, WideStringRight) {
+  CFX_WideString fred(L"FRED");
+  EXPECT_EQ(L"", fred.Right(0));
+  EXPECT_EQ(L"D", fred.Right(1));
+  EXPECT_EQ(L"ED", fred.Right(2));
+  EXPECT_EQ(L"RED", fred.Right(3));
+  EXPECT_EQ(L"FRED", fred.Right(4));
+
+  EXPECT_EQ(L"FRED", fred.Right(5));
+  EXPECT_EQ(L"", fred.Right(-1));
+
+  CFX_WideString empty;
+  EXPECT_EQ(L"", empty.Right(0));
+  EXPECT_EQ(L"", empty.Right(1));
+  EXPECT_EQ(L"", empty.Right(-1));
+}
+
+TEST(fxcrt, WideStringUpperLower) {
+  CFX_WideString fred(L"F-Re.42D");
+  fred.MakeLower();
+  EXPECT_EQ(L"f-re.42d", fred);
+  fred.MakeUpper();
+  EXPECT_EQ(L"F-RE.42D", fred);
+
+  CFX_WideString empty;
+  empty.MakeLower();
+  EXPECT_EQ(L"", empty);
+  empty.MakeUpper();
+  EXPECT_EQ(L"", empty);
+}
+
+TEST(fxcrt, WideStringTrimRight) {
+  CFX_WideString fred(L"  FRED  ");
+  fred.TrimRight();
+  EXPECT_EQ(L"  FRED", fred);
+  fred.TrimRight(L'E');
+  EXPECT_EQ(L"  FRED", fred);
+  fred.TrimRight(L'D');
+  EXPECT_EQ(L"  FRE", fred);
+  fred.TrimRight(L"ERP");
+  EXPECT_EQ(L"  F", fred);
+
+  CFX_WideString blank(L"   ");
+  blank.TrimRight(L"ERP");
+  EXPECT_EQ(L"   ", blank);
+  blank.TrimRight(L'E');
+  EXPECT_EQ(L"   ", blank);
+  blank.TrimRight();
+  EXPECT_EQ(L"", blank);
+
+  CFX_WideString empty;
+  empty.TrimRight(L"ERP");
+  EXPECT_EQ(L"", empty);
+  empty.TrimRight(L'E');
+  EXPECT_EQ(L"", empty);
+  empty.TrimRight();
+  EXPECT_EQ(L"", empty);
+}
+
+TEST(fxcrt, WideStringTrimLeft) {
+  CFX_WideString fred(L"  FRED  ");
+  fred.TrimLeft();
+  EXPECT_EQ(L"FRED  ", fred);
+  fred.TrimLeft(L'E');
+  EXPECT_EQ(L"FRED  ", fred);
+  fred.TrimLeft(L'F');
+  EXPECT_EQ(L"RED  ", fred);
+  fred.TrimLeft(L"ERP");
+  EXPECT_EQ(L"D  ", fred);
+
+  CFX_WideString blank(L"   ");
+  blank.TrimLeft(L"ERP");
+  EXPECT_EQ(L"   ", blank);
+  blank.TrimLeft(L'E');
+  EXPECT_EQ(L"   ", blank);
+  blank.TrimLeft();
+  EXPECT_EQ(L"", blank);
+
+  CFX_WideString empty;
+  empty.TrimLeft(L"ERP");
+  EXPECT_EQ(L"", empty);
+  empty.TrimLeft(L'E');
+  EXPECT_EQ(L"", empty);
+  empty.TrimLeft();
+  EXPECT_EQ(L"", empty);
+}
+
 TEST(fxcrt, WideStringUTF16LE_Encode) {
   struct UTF16LEEncodeCase {
     CFX_WideString ws;