diff --git a/util/strings.go b/util/strings.go
index d7e54e85..ebb19f2b 100644
--- a/util/strings.go
+++ b/util/strings.go
@@ -1,6 +1,7 @@
 package util
 
 import (
+	"errors"
 	"strings"
 )
 
@@ -36,3 +37,35 @@ func FindFirstEOL(s string) int {
 func FindLastSOL(s string) int {
 	return strings.LastIndex(s, "\n") + 1
 }
+
+var (
+	IndexOutOfRange = errors.New("substring out of range")
+)
+
+// SubStringByRune returns the range of the i-th rune (inclusive) through the
+// j-th rune (exclusive) in s.
+func SubstringByRune(s string, low, high int) (string, error) {
+	if low > high || low < 0 || high < 0 {
+		return "", IndexOutOfRange
+	}
+	var bLow, bHigh, j int
+	for i := range s {
+		if j == low {
+			bLow = i
+		}
+		if j == high {
+			bHigh = i
+		}
+		j++
+	}
+	if j < high {
+		return "", IndexOutOfRange
+	}
+	if low == high {
+		return "", nil
+	}
+	if j == high {
+		bHigh = len(s)
+	}
+	return s[bLow:bHigh], nil
+}
diff --git a/util/strings_test.go b/util/strings_test.go
index 58c3de73..6c5cab4b 100644
--- a/util/strings_test.go
+++ b/util/strings_test.go
@@ -20,3 +20,29 @@ func TestFindContext(t *testing.T) {
 		}
 	}
 }
+
+var SubstringByRuneTests = []struct {
+	s         string
+	low, high int
+	wantedStr string
+	wantedErr error
+}{
+	{"Hello world", 1, 4, "ell", nil},
+	{"你好世界", 0, 0, "", nil},
+	{"你好世界", 1, 1, "", nil},
+	{"你好世界", 1, 2, "好", nil},
+	{"你好世界", 1, 4, "好世界", nil},
+	{"你好世界", -1, -1, "", IndexOutOfRange},
+	{"你好世界", 0, 5, "", IndexOutOfRange},
+	{"你好世界", 5, 5, "", IndexOutOfRange},
+}
+
+func TestSubstringByRune(t *testing.T) {
+	for _, tt := range SubstringByRuneTests {
+		s, e := SubstringByRune(tt.s, tt.low, tt.high)
+		if s != tt.wantedStr || e != tt.wantedErr {
+			t.Errorf("SubstringByRune(%q, %v, %d) => (%q, %v), want (%q, %v)",
+				tt.s, tt.low, tt.high, s, e, tt.wantedStr, tt.wantedErr)
+		}
+	}
+}