diff --git a/src/pkg/strings/strings.go b/src/pkg/strings/strings.go
index b6d84d07aa..c192b1826e 100644
--- a/src/pkg/strings/strings.go
+++ b/src/pkg/strings/strings.go
@@ -328,43 +328,24 @@ func ToTitleSpecial(_case unicode.SpecialCase, s string) string {
 // TrimLeftFunc returns a slice of the string s with all leading
 // Unicode code points c satisfying f(c) removed.
 func TrimLeftFunc(s string, f func(r int) bool) string {
-	start, end := 0, len(s)
-	for start < end {
-		wid := 1
-		rune := int(s[start])
-		if rune >= utf8.RuneSelf {
-			rune, wid = utf8.DecodeRuneInString(s[start:end])
-		}
-		if !f(rune) {
-			return s[start:]
-		}
-		start += wid
+	i := indexFunc(s, f, false)
+	if i == -1 {
+		return ""
 	}
-	return s[start:]
+	return s[i:]
 }
 
 // TrimRightFunc returns a slice of the string s with all trailing
 // Unicode code points c satisfying f(c) removed.
 func TrimRightFunc(s string, f func(r int) bool) string {
-	start, end := 0, len(s)
-	for start < end {
-		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; start <= end-wid && !utf8.RuneStart(s[end-wid]); wid++ {
-			}
-			if start > end-wid { // invalid UTF-8 sequence; stop processing
-				return s[start:end]
-			}
-			rune, wid = utf8.DecodeRuneInString(s[end-wid : end])
-		}
-		if !f(rune) {
-			return s[0:end]
-		}
-		end -= wid
+	i := lastIndexFunc(s, f, false)
+	if i >= 0 && s[i] >= utf8.RuneSelf {
+		_, wid := utf8.DecodeRuneInString(s[i:])
+		i += wid
+	} else {
+		i++
 	}
-	return s[0:end]
+	return s[0:i]
 }
 
 // TrimFunc returns a slice of the string s with all leading
@@ -373,6 +354,75 @@ func TrimFunc(s string, f func(r int) bool) string {
 	return TrimRightFunc(TrimLeftFunc(s, f), f)
 }
 
+// IndexFunc returns the index into s of the first Unicode
+// code point satisfying f(c), or -1 if none do.
+func IndexFunc(s string, f func(r int) bool) int {
+	return indexFunc(s, f, true)
+}
+
+// LastIndexFunc returns the index into s of the last
+// Unicode code point satisfying f(c), or -1 if none do.
+func LastIndexFunc(s string, 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. We could use IndexFunc directly, but this
+// way saves a closure allocation.
+func indexFunc(s string, 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.DecodeRuneInString(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. We could use IndexFunc directly, but this
+// way saves a closure allocation.
+func lastIndexFunc(s string, 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.DecodeRuneInString(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 { return IndexRune(cutset, rune) != -1 }
 }
diff --git a/src/pkg/strings/strings_test.go b/src/pkg/strings/strings_test.go
index 2c99a6ec36..e4134d8d67 100644
--- a/src/pkg/strings/strings_test.go
+++ b/src/pkg/strings/strings_test.go
@@ -283,8 +283,14 @@ 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 ☺"},
 }
 
 func tenRunes(rune int) string {
@@ -407,8 +413,28 @@ var trimTests = []TrimTest{
 	TrimTest{TrimRight, "abba", "", "abba"},
 	TrimTest{TrimRight, "", "123", ""},
 	TrimTest{TrimRight, "", "", ""},
+	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)
@@ -426,25 +452,100 @@ 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)
+			}
+		}
 	}
 }
 
+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 predicate struct {
+	f    func(r int) bool
+	name string
+}
+
 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 := TrimFunc(tc.in, tc.f)
+		actual := TrimFunc(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},
+
+	// broken unicode tests
+	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(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(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)
 		}
 	}
 }