diff --git a/src/pkg/bytes/bytes.go b/src/pkg/bytes/bytes.go
index dc0a41553c2..c0ee44ff96c 100644
--- a/src/pkg/bytes/bytes.go
+++ b/src/pkg/bytes/bytes.go
@@ -127,7 +127,7 @@ func LastIndex(s, sep []byte) int {
 	return -1
 }
 
-// IndexAny interprets s as a sequence of UTF-8 encoded Unicode code points.
+// IndexAny interprets s as a sequence of UTF-8-encoded Unicode code points.
 // It returns the byte index of the first occurrence in s of any of the Unicode
 // code points in chars.  It returns -1 if chars is empty or if there is no code
 // point in common.
@@ -278,7 +278,7 @@ func HasSuffix(s, suffix []byte) bool {
 // Map returns a copy of the byte array s with all its characters modified
 // according to the mapping function. If mapping returns a negative value, the character is
 // dropped from the string with no replacement.  The characters in s and the
-// output are interpreted as UTF-8 encoded Unicode code points.
+// output are interpreted as UTF-8-encoded Unicode code points.
 func Map(mapping func(rune int) int, s []byte) []byte {
 	// In the worst case, the array can grow when mapped, making
 	// things unpleasant.  But it's so rare we barge in assuming it's
@@ -378,52 +378,104 @@ func Title(s []byte) []byte {
 		s)
 }
 
-// TrimLeftFunc returns a subslice of s by slicing off all leading UTF-8 encoded
+// TrimLeftFunc returns a subslice of s by slicing off all leading UTF-8-encoded
 // Unicode code points c that satisfy f(c).
 func TrimLeftFunc(s []byte, f func(r int) bool) []byte {
-	var start, wid int
-	for start = 0; start < len(s); start += wid {
-		wid = 1
-		rune := int(s[start])
-		if rune >= utf8.RuneSelf {
-			rune, wid = utf8.DecodeRune(s[start:])
-		}
-		if !f(rune) {
-			break
-		}
+	i := indexFunc(s, f, false)
+	if i == -1 {
+		return nil
 	}
-	return s[start:]
+	return s[i:]
 }
 
 // TrimRightFunc returns a subslice of s by slicing off all trailing UTF-8
 // encoded Unicode code points c that satisfy f(c).
 func TrimRightFunc(s []byte, f func(r int) bool) []byte {
-	var end, wid int
-	for end = len(s); end > 0; end -= wid {
-		wid = 1
-		rune := int(s[end-wid])
-		if rune >= utf8.RuneSelf {
-			// Back up & look for beginning of rune. Mustn't pass start.
-			for wid = 2; end-wid >= 0 && !utf8.RuneStart(s[end-wid]); wid++ {
-			}
-			if end-wid < 0 { // invalid UTF-8 sequence; stop processing
-				break
-			}
-			rune, wid = utf8.DecodeRune(s[end-wid : end])
-		}
-		if !f(rune) {
-			break
-		}
+	i := lastIndexFunc(s, f, false)
+	if i >= 0 && s[i] >= utf8.RuneSelf {
+		_, wid := utf8.DecodeRune(s[i:])
+		i += wid
+	} else {
+		i++
 	}
-	return s[0:end]
+	return s[0:i]
 }
 
 // TrimFunc returns a subslice of s by slicing off all leading and trailing
-// UTF-8 encoded Unicode code points c that satisfy f(c).
+// UTF-8-encoded Unicode code points c that satisfy f(c).
 func TrimFunc(s []byte, f func(r int) bool) []byte {
 	return TrimRightFunc(TrimLeftFunc(s, f), f)
 }
 
+// IndexFunc interprets s as a sequence of UTF-8-encoded Unicode code points.
+// It returns the byte index in s of the first Unicode
+// code point satisfying f(c), or -1 if none do.
+func IndexFunc(s []byte, f func(r int) bool) int {
+	return indexFunc(s, f, true)
+}
+
+// LastIndexFunc interprets s as a sequence of UTF-8-encoded Unicode code points.
+// It returns the byte index in s of the last Unicode
+// code point satisfying f(c), or -1 if none do.
+func LastIndexFunc(s []byte, f func(r int) bool) int {
+	return lastIndexFunc(s, f, true)
+}
+
+// indexFunc is the same as IndexFunc except that if
+// truth==false, the sense of the predicate function is
+// inverted.
+func indexFunc(s []byte, f func(r int) bool, truth bool) int {
+	start := 0
+	for start < len(s) {
+		wid := 1
+		rune := int(s[start])
+		if rune >= utf8.RuneSelf {
+			rune, wid = utf8.DecodeRune(s[start:])
+		}
+		if f(rune) == truth {
+			return start
+		}
+		start += wid
+	}
+	return -1
+}
+
+// lastIndexFunc is the same as LastIndexFunc except that if
+// truth==false, the sense of the predicate function is
+// inverted.
+func lastIndexFunc(s []byte, f func(r int) bool, truth bool) int {
+	end := len(s)
+	for end > 0 {
+		start := end - 1
+		rune := int(s[start])
+		if rune >= utf8.RuneSelf {
+			// Back up & look for beginning of rune. Mustn't pass start.
+			for start--; start >= 0; start-- {
+				if utf8.RuneStart(s[start]) {
+					break
+				}
+			}
+			if start < 0 {
+				return -1
+			}
+			var wid int
+			rune, wid = utf8.DecodeRune(s[start:end])
+
+			// If we've decoded fewer bytes than we expected,
+			// we've got some invalid UTF-8, so make sure we return
+			// the last possible index in s.
+			if start+wid < end && f(utf8.RuneError) == truth {
+				return end - 1
+			}
+		}
+		if f(rune) == truth {
+			return start
+		}
+		end = start
+	}
+	return -1
+}
+
 func makeCutsetFunc(cutset string) func(rune int) bool {
 	return func(rune int) bool {
 		for _, c := range cutset {
@@ -436,19 +488,19 @@ func makeCutsetFunc(cutset string) func(rune int) bool {
 }
 
 // Trim returns a subslice of s by slicing off all leading and
-// trailing UTF-8 encoded Unicode code points contained in cutset.
+// trailing UTF-8-encoded Unicode code points contained in cutset.
 func Trim(s []byte, cutset string) []byte {
 	return TrimFunc(s, makeCutsetFunc(cutset))
 }
 
 // TrimLeft returns a subslice of s by slicing off all leading
-// UTF-8 encoded Unicode code points contained in cutset.
+// UTF-8-encoded Unicode code points contained in cutset.
 func TrimLeft(s []byte, cutset string) []byte {
 	return TrimLeftFunc(s, makeCutsetFunc(cutset))
 }
 
 // TrimRight returns a subslice of s by slicing off all trailing
-// UTF-8 encoded Unicode code points that are contained in cutset.
+// UTF-8-encoded Unicode code points that are contained in cutset.
 func TrimRight(s []byte, cutset string) []byte {
 	return TrimRightFunc(s, makeCutsetFunc(cutset))
 }
diff --git a/src/pkg/bytes/bytes_test.go b/src/pkg/bytes/bytes_test.go
index de5edd120fc..5d3e9f66a1e 100644
--- a/src/pkg/bytes/bytes_test.go
+++ b/src/pkg/bytes/bytes_test.go
@@ -8,6 +8,7 @@ import (
 	. "bytes"
 	"testing"
 	"unicode"
+	"utf8"
 )
 
 func eq(a, b []string) bool {
@@ -367,8 +368,14 @@ var trimSpaceTests = []StringTest{
 	StringTest{" \t\r\n x\t\t\r\r\n\n ", "x"},
 	StringTest{" \u2000\t\r\n x\t\t\r\r\ny\n \u3000", "x\t\t\r\r\ny"},
 	StringTest{"1 \t\r\n2", "1 \t\r\n2"},
-	StringTest{" x\x80", "x\x80"}, // invalid UTF-8 on end
-	StringTest{" x\xc0", "x\xc0"}, // invalid UTF-8 on end
+	StringTest{" x\x80", "x\x80"},
+	StringTest{" x\xc0", "x\xc0"},
+	StringTest{"x \xc0\xc0 ", "x \xc0\xc0"},
+	StringTest{"x \xc0", "x \xc0"},
+	StringTest{"x \xc0 ", "x \xc0"},
+	StringTest{"x \xc0\xc0 ", "x \xc0\xc0"},
+	StringTest{"x ☺\xc0\xc0 ", "x ☺\xc0\xc0"},
+	StringTest{"x ☺ ", "x ☺"},
 }
 
 // Bytes returns a new slice containing the bytes in s.
@@ -607,6 +614,7 @@ var trimTests = []TrimTest{
 	TrimTest{TrimRight, "abba", "", "abba"},
 	TrimTest{TrimRight, "", "123", ""},
 	TrimTest{TrimRight, "", "", ""},
+	TrimTest{TrimRight, "☺\xc0", "☺", "☺\xc0"},
 }
 
 func TestTrim(t *testing.T) {
@@ -629,22 +637,90 @@ func TestTrim(t *testing.T) {
 	}
 }
 
+type predicate struct {
+	f    func(r int) bool
+	name string
+}
+
+var isSpace = predicate{unicode.IsSpace, "IsSpace"}
+var isDigit = predicate{unicode.IsDigit, "IsDigit"}
+var isUpper = predicate{unicode.IsUpper, "IsUpper"}
+var isValidRune = predicate{
+	func(r int) bool {
+		return r != utf8.RuneError
+	},
+	"IsValidRune",
+}
+
 type TrimFuncTest struct {
-	f             func(r int) bool
-	name, in, out string
+	f       predicate
+	in, out string
+}
+
+func not(p predicate) predicate {
+	return predicate{
+		func(r int) bool {
+			return !p.f(r)
+		},
+		"not " + p.name,
+	}
 }
 
 var trimFuncTests = []TrimFuncTest{
-	TrimFuncTest{unicode.IsSpace, "IsSpace", space + " hello " + space, "hello"},
-	TrimFuncTest{unicode.IsDigit, "IsDigit", "\u0e50\u0e5212hello34\u0e50\u0e51", "hello"},
-	TrimFuncTest{unicode.IsUpper, "IsUpper", "\u2C6F\u2C6F\u2C6F\u2C6FABCDhelloEF\u2C6F\u2C6FGH\u2C6F\u2C6F", "hello"},
+	TrimFuncTest{isSpace, space + " hello " + space, "hello"},
+	TrimFuncTest{isDigit, "\u0e50\u0e5212hello34\u0e50\u0e51", "hello"},
+	TrimFuncTest{isUpper, "\u2C6F\u2C6F\u2C6F\u2C6FABCDhelloEF\u2C6F\u2C6FGH\u2C6F\u2C6F", "hello"},
+	TrimFuncTest{not(isSpace), "hello" + space + "hello", space},
+	TrimFuncTest{not(isDigit), "hello\u0e50\u0e521234\u0e50\u0e51helo", "\u0e50\u0e521234\u0e50\u0e51"},
+	TrimFuncTest{isValidRune, "ab\xc0a\xc0cd", "\xc0a\xc0"},
+	TrimFuncTest{not(isValidRune), "\xc0a\xc0", "a"},
 }
 
 func TestTrimFunc(t *testing.T) {
 	for _, tc := range trimFuncTests {
-		actual := string(TrimFunc([]byte(tc.in), tc.f))
+		actual := string(TrimFunc([]byte(tc.in), tc.f.f))
 		if actual != tc.out {
-			t.Errorf("TrimFunc(%q, %q) = %q; want %q", tc.in, tc.name, actual, tc.out)
+			t.Errorf("TrimFunc(%q, %q) = %q; want %q", tc.in, tc.f.name, actual, tc.out)
+		}
+	}
+}
+
+type IndexFuncTest struct {
+	in          string
+	f           predicate
+	first, last int
+}
+
+var indexFuncTests = []IndexFuncTest{
+	IndexFuncTest{"", isValidRune, -1, -1},
+	IndexFuncTest{"abc", isDigit, -1, -1},
+	IndexFuncTest{"0123", isDigit, 0, 3},
+	IndexFuncTest{"a1b", isDigit, 1, 1},
+	IndexFuncTest{space, isSpace, 0, len(space) - 3}, // last rune in space is 3 bytes
+	IndexFuncTest{"\u0e50\u0e5212hello34\u0e50\u0e51", isDigit, 0, 18},
+	IndexFuncTest{"\u2C6F\u2C6F\u2C6F\u2C6FABCDhelloEF\u2C6F\u2C6FGH\u2C6F\u2C6F", isUpper, 0, 34},
+	IndexFuncTest{"12\u0e50\u0e52hello34\u0e50\u0e51", not(isDigit), 8, 12},
+
+	// tests of invalid UTF-8
+	IndexFuncTest{"\x801", isDigit, 1, 1},
+	IndexFuncTest{"\x80abc", isDigit, -1, -1},
+	IndexFuncTest{"\xc0a\xc0", isValidRune, 1, 1},
+	IndexFuncTest{"\xc0a\xc0", not(isValidRune), 0, 2},
+	IndexFuncTest{"\xc0☺\xc0", not(isValidRune), 0, 4},
+	IndexFuncTest{"\xc0☺\xc0\xc0", not(isValidRune), 0, 5},
+	IndexFuncTest{"ab\xc0a\xc0cd", not(isValidRune), 2, 4},
+	IndexFuncTest{"a\xe0\x80cd", not(isValidRune), 1, 2},
+}
+
+func TestIndexFunc(t *testing.T) {
+	for _, tc := range indexFuncTests {
+		first := IndexFunc([]byte(tc.in), tc.f.f)
+		if first != tc.first {
+			t.Errorf("IndexFunc(%q, %s) = %d; want %d", tc.in, tc.f.name, first, tc.first)
+		}
+		last := LastIndexFunc([]byte(tc.in), tc.f.f)
+		if last != tc.last {
+			t.Errorf("LastIndexFunc(%q, %s) = %d; want %d", tc.in, tc.f.name, last, tc.last)
 		}
 	}
 }
diff --git a/src/pkg/strings/strings.go b/src/pkg/strings/strings.go
index 594f03527e1..925566c744e 100644
--- a/src/pkg/strings/strings.go
+++ b/src/pkg/strings/strings.go
@@ -422,8 +422,7 @@ func LastIndexFunc(s string, f func(r int) bool) int {
 
 // indexFunc is the same as IndexFunc except that if
 // truth==false, the sense of the predicate function is
-// inverted. We could use IndexFunc directly, but this
-// way saves a closure allocation.
+// inverted.
 func indexFunc(s string, f func(r int) bool, truth bool) int {
 	start := 0
 	for start < len(s) {
@@ -442,8 +441,7 @@ func indexFunc(s string, f func(r int) bool, truth bool) int {
 
 // lastIndexFunc is the same as LastIndexFunc except that if
 // truth==false, the sense of the predicate function is
-// inverted. We could use IndexFunc directly, but this
-// way saves a closure allocation.
+// inverted.
 func lastIndexFunc(s string, f func(r int) bool, truth bool) int {
 	end := len(s)
 	for end > 0 {
diff --git a/src/pkg/strings/strings_test.go b/src/pkg/strings/strings_test.go
index 8b0c98113a6..9e8feceaa2a 100644
--- a/src/pkg/strings/strings_test.go
+++ b/src/pkg/strings/strings_test.go
@@ -420,25 +420,6 @@ var trimTests = []TrimTest{
 	TrimTest{TrimRight, "☺\xc0", "☺", "☺\xc0"},
 }
 
-// naiveTrimRight implements a version of TrimRight
-// by scanning forwards from the start of s.
-func naiveTrimRight(s string, cutset string) string {
-	i := -1
-	for j, r := range s {
-		if IndexRune(cutset, r) == -1 {
-			i = j
-		}
-	}
-	if i >= 0 && s[i] >= utf8.RuneSelf {
-		_, wid := utf8.DecodeRuneInString(s[i:])
-		i += wid
-	} else {
-		i++
-	}
-	return s[0:i]
-}
-
-
 func TestTrim(t *testing.T) {
 	for _, tc := range trimTests {
 		actual := tc.f(tc.in, tc.cutset)
@@ -456,16 +437,14 @@ func TestTrim(t *testing.T) {
 		if actual != tc.out {
 			t.Errorf("%s(%q, %q) = %q; want %q", name, tc.in, tc.cutset, actual, tc.out)
 		}
-		// test equivalence of TrimRight to naive version
-		if tc.f == TrimRight {
-			naive := naiveTrimRight(tc.in, tc.cutset)
-			if naive != actual {
-				t.Errorf("TrimRight(%q, %q) = %q, want %q", tc.in, tc.cutset, actual, naive)
-			}
-		}
 	}
 }
 
+type predicate struct {
+	f    func(r int) bool
+	name string
+}
+
 var isSpace = predicate{unicode.IsSpace, "IsSpace"}
 var isDigit = predicate{unicode.IsDigit, "IsDigit"}
 var isUpper = predicate{unicode.IsUpper, "IsUpper"}
@@ -476,11 +455,6 @@ var isValidRune = predicate{
 	"IsValidRune",
 }
 
-type predicate struct {
-	f    func(r int) bool
-	name string
-}
-
 type TrimFuncTest struct {
 	f       predicate
 	in, out string
@@ -530,7 +504,7 @@ var indexFuncTests = []IndexFuncTest{
 	IndexFuncTest{"\u2C6F\u2C6F\u2C6F\u2C6FABCDhelloEF\u2C6F\u2C6FGH\u2C6F\u2C6F", isUpper, 0, 34},
 	IndexFuncTest{"12\u0e50\u0e52hello34\u0e50\u0e51", not(isDigit), 8, 12},
 
-	// broken unicode tests
+	// tests of invalid UTF-8
 	IndexFuncTest{"\x801", isDigit, 1, 1},
 	IndexFuncTest{"\x80abc", isDigit, -1, -1},
 	IndexFuncTest{"\xc0a\xc0", isValidRune, 1, 1},