2 files changed, 27 insertions, 0 deletions
diff --git a/src/strings/strings.go b/src/strings/strings.go
index c6085f51d5..7b8a6b536b 100644
--- a/src/strings/strings.go
+++ b/src/strings/strings.go
@@ -271,6 +271,16 @@ func LastIndexAny(s, chars string) int {
 	return -1
 }
 
+// LastIndexByte returns the index of the last instance of c in s, or -1 if c is not present in s.
+func LastIndexByte(s string, c byte) int {
+	for i := len(s) - 1; i >= 0; i-- {
+		if s[i] == c {
+			return i
+		}
+	}
+	return -1
+}
+
 // Generic split: splits after each instance of sep,
 // including sepSave bytes of sep in the subarrays.
 func genSplit(s, sep string, sepSave, n int) []string {
diff --git a/src/strings/strings_test.go b/src/strings/strings_test.go
index ee0c260753..4e21deaecd 100644
--- a/src/strings/strings_test.go
+++ b/src/strings/strings_test.go
@@ -120,6 +120,23 @@ func TestLastIndex(t *testing.T)    { runIndexTests(t, LastIndex, "LastIndex", l
 func TestIndexAny(t *testing.T)     { runIndexTests(t, IndexAny, "IndexAny", indexAnyTests) }
 func TestLastIndexAny(t *testing.T) { runIndexTests(t, LastIndexAny, "LastIndexAny", lastIndexAnyTests) }
 
+func TestLastIndexByte(t *testing.T) {
+	testCases := []IndexTest{
+		{"", "q", -1},
+		{"abcdef", "q", -1},
+		{"abcdefabcdef", "a", len("abcdef")},      // something in the middle
+		{"abcdefabcdef", "f", len("abcdefabcde")}, // last byte
+		{"zabcdefabcdef", "z", 0},                 // first byte
+		{"a☺b☻c☹d", "b", len("a☺")},               // non-ascii
+	}
+	for _, test := range testCases {
+		actual := LastIndexByte(test.s, test.sep[0])
+		if actual != test.out {
+			t.Errorf("LastIndexByte(%q,%c) = %v; want %v", test.s, test.sep[0], actual, test.out)
+		}
+	}
+}
+
 var indexRuneTests = []struct {
 	s    string
 	rune rune