Index: doc/string.n
==================================================================
--- doc/string.n
+++ doc/string.n
@@ -85,10 +85,27 @@
 .RS
 .PP
 If \fIcharIndex\fR is less than 0 or greater than or equal to the
 length of the string then this command returns an empty string.
 .RE
+.TP
+\fBstring insert \fIstring index insertString\fR
+.
+Returns a copy of \fIstring\fR with \fIinsertString\fR inserted at the
+\fIindex\fR'th character.  \fIindex\fR may be specified as described in the
+\fBSTRING INDICES\fR section.
+.RS
+.PP
+If \fIindex\fR is start-relative, the first character inserted in the returned
+string will be at the specified index.  If \fIindex\fR is end-relative, the last
+character inserted in the returned string will be at the specified index.
+.PP
+If \fIindex\fR is at or before the start of \fIstring\fR (e.g., \fIindex\fR is
+\fB0\fR), \fIinsertString\fR is prepended to \fIstring\fR.  If \fIindex\fR is at
+or after the end of \fIstring\fR (e.g., \fIindex\fR is \fBend\fR),
+\fIinsertString\fR is appended to \fIstring\fR.
+.RE
 .TP
 \fBstring is \fIclass\fR ?\fB\-strict\fR? ?\fB\-failindex \fIvarname\fR? \fIstring\fR
 .
 Returns 1 if \fIstring\fR is a valid member of the specified character
 class, otherwise returns 0.  If \fB\-strict\fR is specified, then an

Index: generic/tclCmdMZ.c
==================================================================
--- generic/tclCmdMZ.c
+++ generic/tclCmdMZ.c
@@ -1458,10 +1458,67 @@
 }
 
 /*
  *----------------------------------------------------------------------
  *
+ * StringInsertCmd --
+ *
+ *	This procedure is invoked to process the "string insert" Tcl command.
+ *	See the user documentation for details on what it does. Note that this
+ *	command only functions correctly on properly formed Tcl UTF strings.
+ *
+ * Results:
+ *	A standard Tcl result.
+ *
+ * Side effects:
+ *	See the user documentation.
+ *
+ *----------------------------------------------------------------------
+ */
+
+static int
+StringInsertCmd(
+    ClientData dummy,		/* Not used */
+    Tcl_Interp *interp,		/* Current interpreter */
+    int objc,			/* Number of arguments */
+    Tcl_Obj *const objv[])	/* Argument objects */
+{
+    int length;			/* String length */
+    int index;			/* Insert index */
+    Tcl_Obj *outObj;		/* Output object */
+
+    if (objc != 4) {
+	Tcl_WrongNumArgs(interp, 1, objv, "string index insertString");
+	return TCL_ERROR;
+    }
+
+    length = Tcl_GetCharLength(objv[1]);
+    if (TclGetIntForIndexM(interp, objv[2], length, &index) != TCL_OK) {
+	return TCL_ERROR;
+    }
+
+    if (index < 0) {
+	index = 0;
+    }
+    if (index > length) {
+	index = length;
+    }
+
+    outObj = TclStringReplace(interp, objv[1], index, 0, objv[3],
+	    TCL_STRING_IN_PLACE);
+
+    if (outObj != NULL) {
+	Tcl_SetObjResult(interp, outObj);
+	return TCL_OK;
+    }
+
+    return TCL_ERROR;
+}
+
+/*
+ *----------------------------------------------------------------------
+ *
  * StringIsCmd --
  *
  *	This procedure is invoked to process the "string is" Tcl command. See
  *	the user documentation for details on what it does. Note that this
  *	command only functions correctly on properly formed Tcl UTF strings.
@@ -3267,10 +3324,11 @@
 	{"cat",		StringCatCmd,	TclCompileStringCatCmd, NULL, NULL, 0},
 	{"compare",	StringCmpCmd,	TclCompileStringCmpCmd, NULL, NULL, 0},
 	{"equal",	StringEqualCmd,	TclCompileStringEqualCmd, NULL, NULL, 0},
 	{"first",	StringFirstCmd,	TclCompileStringFirstCmd, NULL, NULL, 0},
 	{"index",	StringIndexCmd,	TclCompileStringIndexCmd, NULL, NULL, 0},
+	{"insert",	StringInsertCmd, TclCompileStringInsertCmd, NULL, NULL, 0},
 	{"is",		StringIsCmd,	TclCompileStringIsCmd, NULL, NULL, 0},
 	{"last",	StringLastCmd,	TclCompileStringLastCmd, NULL, NULL, 0},
 	{"length",	StringLenCmd,	TclCompileStringLenCmd, NULL, NULL, 0},
 	{"map",		StringMapCmd,	TclCompileStringMapCmd, NULL, NULL, 0},
 	{"match",	StringMatchCmd,	TclCompileStringMatchCmd, NULL, NULL, 0},

Index: generic/tclCompCmdsSZ.c
==================================================================
--- generic/tclCompCmdsSZ.c
+++ generic/tclCompCmdsSZ.c
@@ -443,10 +443,67 @@
     tokenPtr = TokenAfter(parsePtr->tokenPtr);
     CompileWord(envPtr, tokenPtr, interp, 1);
     tokenPtr = TokenAfter(tokenPtr);
     CompileWord(envPtr, tokenPtr, interp, 2);
     TclEmitOpcode(INST_STR_INDEX, envPtr);
+    return TCL_OK;
+}
+
+int
+TclCompileStringInsertCmd(
+    Tcl_Interp *interp,		/* Used for error reporting. */
+    Tcl_Parse *parsePtr,	/* Points to a parse structure for the command
+				 * created by Tcl_ParseCommand. */
+    Command *cmdPtr,		/* Points to defintion of command being
+				 * compiled. */
+    CompileEnv *envPtr)		/* Holds resulting instructions. */
+{
+    Tcl_Token *tokenPtr;
+    DefineLineInformation;	/* TIP #280 */
+    int idx;
+
+    if (parsePtr->numWords != 4) {
+	return TCL_ERROR;
+    }
+
+    /* Compute and push the string in which to insert */
+    tokenPtr = TokenAfter(parsePtr->tokenPtr);
+    CompileWord(envPtr, tokenPtr, interp, 1);
+
+    /* See what can be discovered about index at compile time */
+    tokenPtr = TokenAfter(tokenPtr);
+    if (TCL_OK != TclGetIndexFromToken(tokenPtr, TCL_INDEX_START,
+	    TCL_INDEX_END, &idx)) {
+
+	/* Nothing useful knowable - cease compile; let it direct eval */
+	return TCL_OK;
+    }
+
+    /* Compute and push the string to be inserted */
+    tokenPtr = TokenAfter(tokenPtr);
+    CompileWord(envPtr, tokenPtr, interp, 3);
+
+    if (idx == TCL_INDEX_START) {
+	/* Prepend the insertion string */
+	OP4(	REVERSE, 2);
+	OP1(	STR_CONCAT1, 2);
+    } else  if (idx == TCL_INDEX_END) {
+	/* Append the insertion string */
+	OP1(	STR_CONCAT1, 2);
+    } else {
+	/* Prefix + insertion + suffix */
+	if (idx < TCL_INDEX_END) {
+	    /* See comments in compiler for [linsert]. */
+	    idx++;
+	}
+	OP4(	OVER, 1);
+	OP44(	STR_RANGE_IMM, 0, idx-1);
+	OP4(	REVERSE, 3);
+	OP44(	STR_RANGE_IMM, idx, TCL_INDEX_END);
+	OP1(	STR_CONCAT1, 3);
+    }
+
     return TCL_OK;
 }
 
 int
 TclCompileStringIsCmd(

Index: generic/tclInt.h
==================================================================
--- generic/tclInt.h
+++ generic/tclInt.h
@@ -3804,10 +3804,13 @@
 MODULE_SCOPE int	TclCompileStringFirstCmd(Tcl_Interp *interp,
 			    Tcl_Parse *parsePtr, Command *cmdPtr,
 			    struct CompileEnv *envPtr);
 MODULE_SCOPE int	TclCompileStringIndexCmd(Tcl_Interp *interp,
 			    Tcl_Parse *parsePtr, Command *cmdPtr,
+			    struct CompileEnv *envPtr);
+MODULE_SCOPE int	TclCompileStringInsertCmd(Tcl_Interp *interp,
+			    Tcl_Parse *parsePtr, Command *cmdPtr,
 			    struct CompileEnv *envPtr);
 MODULE_SCOPE int	TclCompileStringIsCmd(Tcl_Interp *interp,
 			    Tcl_Parse *parsePtr, Command *cmdPtr,
 			    struct CompileEnv *envPtr);
 MODULE_SCOPE int	TclCompileStringLastCmd(Tcl_Interp *interp,

Index: generic/tclStringObj.c
==================================================================
--- generic/tclStringObj.c
+++ generic/tclStringObj.c
@@ -3862,11 +3862,12 @@
 /*
  *---------------------------------------------------------------------------
  *
  * TclStringReplace --
  *
- *	Implements the inner engine of the [string replace] command.
+ *	Implements the inner engine of the [string replace] and
+ *	[string insert] commands.
  *
  *	The result is a concatenation of a prefix from objPtr, characters
  *	0 through first-1, the insertPtr string value, and a suffix from
  *	objPtr, characters from first + count to the end. The effect is as if
  *	the inner substring of characters first through first+count-1 are
@@ -3909,11 +3910,11 @@
 	}
     }
 
     /*
      * The caller very likely had to call Tcl_GetCharLength() or similar
-     * to be able to process index values.  This means it is like that
+     * to be able to process index values.  This means it is likely that
      * objPtr is either a proper "bytearray" or a "string" or else it has
      * a known and short string rep.
      */
 
     if (TclIsPureByteArray(objPtr)) {

Index: tests/string.test
==================================================================
--- tests/string.test
+++ tests/string.test
@@ -71,11 +71,11 @@
 }
 
 
 test string-1.1.$noComp {error conditions} {
     list [catch {run {string gorp a b}} msg] $msg
-} {1 {unknown or ambiguous subcommand "gorp": must be bytelength, cat, compare, equal, first, index, is, last, length, map, match, range, repeat, replace, reverse, tolower, totitle, toupper, trim, trimleft, trimright, wordend, or wordstart}}
+} {1 {unknown or ambiguous subcommand "gorp": must be bytelength, cat, compare, equal, first, index, insert, is, last, length, map, match, range, repeat, replace, reverse, tolower, totitle, toupper, trim, trimleft, trimright, wordend, or wordstart}}
 test string-1.2.$noComp {error conditions} {
     list [catch {run {string}} msg] $msg
 } {1 {wrong # args: should be "string subcommand ?arg ...?"}}
 test stringComp-1.3.$noComp {error condition - undefined method during compile} {
     # We don't want this to complain about 'never' because it may never
@@ -1798,11 +1798,11 @@
 test string-20.1.$noComp {string trimright errors} {
     list [catch {run {string trimright}} msg] $msg
 } {1 {wrong # args: should be "string trimright string ?chars?"}}
 test string-20.2.$noComp {string trimright errors} {
     list [catch {run {string trimg a}} msg] $msg
-} {1 {unknown or ambiguous subcommand "trimg": must be bytelength, cat, compare, equal, first, index, is, last, length, map, match, range, repeat, replace, reverse, tolower, totitle, toupper, trim, trimleft, trimright, wordend, or wordstart}}
+} {1 {unknown or ambiguous subcommand "trimg": must be bytelength, cat, compare, equal, first, index, insert, is, last, length, map, match, range, repeat, replace, reverse, tolower, totitle, toupper, trim, trimleft, trimright, wordend, or wordstart}}
 test string-20.3.$noComp {string trimright} {
     run {string trimright "    XYZ      "}
 } {    XYZ}
 test string-20.4.$noComp {string trimright} {
     run {string trimright "   "}
@@ -1857,11 +1857,11 @@
     run {string wordend "\uc700\uc700 abc" 8}
 } 6
 
 test string-22.1.$noComp {string wordstart} {
     list [catch {run {string word a}} msg] $msg
-} {1 {unknown or ambiguous subcommand "word": must be bytelength, cat, compare, equal, first, index, is, last, length, map, match, range, repeat, replace, reverse, tolower, totitle, toupper, trim, trimleft, trimright, wordend, or wordstart}}
+} {1 {unknown or ambiguous subcommand "word": must be bytelength, cat, compare, equal, first, index, insert, is, last, length, map, match, range, repeat, replace, reverse, tolower, totitle, toupper, trim, trimleft, trimright, wordend, or wordstart}}
 test string-22.2.$noComp {string wordstart} {
     list [catch {run {string wordstart a}} msg] $msg
 } {1 {wrong # args: should be "string wordstart string index"}}
 test string-22.3.$noComp {string wordstart} {
     list [catch {run {string wordstart a b c}} msg] $msg
@@ -2329,10 +2329,93 @@
     run {string cat [set data [binary format a* hello]] [encoding convertto $data] [unset data]}
 } hellohello
 test string-30.1.2.$noComp {[Bug ba921a8d98]: inplace cat by subst (compiled to "strcat" instruction)} {
     run {set x "[set data [binary format a* hello]][encoding convertto $data][unset data]"}
 } hellohello
+
+# Note: string-31.* tests use [tcl::string::insert] rather than [string insert]
+# to dodge ticket [3397978fff] which would cause all arguments to be shared,
+# thereby preventing the optimizations from being tested.
+test string-31.1.$noComp {string insert, start of string} {
+    run {tcl::string::insert 0123 0 _}
+} _0123
+test string-31.2.$noComp {string insert, middle of string} {
+    run {tcl::string::insert 0123 2 _}
+} 01_23
+test string-31.3.$noComp {string insert, end of string} {
+    run {tcl::string::insert 0123 4 _}
+} 0123_
+test string-31.4.$noComp {string insert, start of string, end-relative} {
+    run {tcl::string::insert 0123 end-4 _}
+} _0123
+test string-31.5.$noComp {string insert, middle of string, end-relative} {
+    run {tcl::string::insert 0123 end-2 _}
+} 01_23
+test string-31.6.$noComp {string insert, end of string, end-relative} {
+    run {tcl::string::insert 0123 end _}
+} 0123_
+test string-31.7.$noComp {string insert, empty target string} {
+    run {tcl::string::insert {} 0 _}
+} _
+test string-31.8.$noComp {string insert, empty insert string} {
+    run {tcl::string::insert 0123 0 {}}
+} 0123
+test string-31.9.$noComp {string insert, empty strings} {
+    run {tcl::string::insert {} 0 {}}
+} {}
+test string-31.10.$noComp {string insert, negative index} {
+    run {tcl::string::insert 0123 -1 _}
+} _0123
+test string-31.11.$noComp {string insert, index beyond end} {
+    run {tcl::string::insert 0123 5 _}
+} 0123_
+test string-31.12.$noComp {string insert, start of string, pure byte array} {
+    run {tcl::string::insert [makeByteArray 0123] 0 [makeByteArray _]}
+} _0123
+test string-31.13.$noComp {string insert, middle of string, pure byte array} {
+    run {tcl::string::insert [makeByteArray 0123] 2 [makeByteArray _]}
+} 01_23
+test string-31.14.$noComp {string insert, end of string, pure byte array} {
+    run {tcl::string::insert [makeByteArray 0123] 4 [makeByteArray _]}
+} 0123_
+test string-31.15.$noComp {string insert, pure byte array, neither shared} {
+    run {tcl::string::insert [makeByteArray 0123] 2 [makeByteArray _]}
+} 01_23
+test string-31.16.$noComp {string insert, pure byte array, first shared} {
+    run {tcl::string::insert [makeShared [makeByteArray 0123]] 2\
+            [makeByteArray _]}
+} 01_23
+test string-31.17.$noComp {string insert, pure byte array, second shared} {
+    run {tcl::string::insert [makeByteArray 0123] 2\
+            [makeShared [makeByteArray _]]}
+} 01_23
+test string-31.18.$noComp {string insert, pure byte array, both shared} {
+    run {tcl::string::insert [makeShared [makeByteArray 0123]] 2\
+            [makeShared [makeByteArray _]]}
+} 01_23
+test string-31.19.$noComp {string insert, start of string, pure Unicode} {
+    run {tcl::string::insert [makeUnicode 0123] 0 [makeUnicode _]}
+} _0123
+test string-31.20.$noComp {string insert, middle of string, pure Unicode} {
+    run {tcl::string::insert [makeUnicode 0123] 2 [makeUnicode _]}
+} 01_23
+test string-31.21.$noComp {string insert, end of string, pure Unicode} {
+    run {tcl::string::insert [makeUnicode 0123] 4 [makeUnicode _]}
+} 0123_
+test string-31.22.$noComp {string insert, str start, pure Uni, first shared} {
+    run {tcl::string::insert [makeShared [makeUnicode 0123]] 0 [makeUnicode _]}
+} _0123
+test string-31.23.$noComp {string insert, string mid, pure Uni, 2nd shared} {
+    run {tcl::string::insert [makeUnicode 0123] 2 [makeShared [makeUnicode _]]}
+} 01_23
+test string-31.24.$noComp {string insert, string end, pure Uni, both shared} {
+    run {tcl::string::insert [makeShared [makeUnicode 0123]] 4\
+            [makeShared [makeUnicode _]]}
+} 0123_
+test string-31.25.$noComp {string insert, neither byte array nor Unicode} {
+    run {tcl::string::insert [makeList a b c] 1 zzzzzz}
+} {azzzzzz b c}
 
 test string-31.1.$noComp {string is dict} {
     string is dict {a b c d}
 } 1
 test string-31.1a.$noComp {string is dict} {