a.diff

diff --git a/core/src/main/java/org/jruby/RubyString.java b/core/src/main/java/org/jruby/RubyString.java
index a1a06fefba..deb9ccd27d 100644
--- a/core/src/main/java/org/jruby/RubyString.java
+++ b/core/src/main/java/org/jruby/RubyString.java
@@ -3208,7 +3208,7 @@ public class RubyString extends RubyObject implements CharSequence, EncodingCapa
         final int patternLen = pattern.size();
         final Encoding patternEnc = this.checkEncoding(pattern);
 
-        int beg = StringSupport.index(this, pattern, 0, patternEnc);
+        int beg = StringSupport.index(getByteList(), pattern.getByteList(), 0, patternEnc);
         int begz;
         if (beg < 0) {
             if (useBackref) context.setBackRef(context.nil);
@@ -3258,7 +3258,7 @@ public class RubyString extends RubyObject implements CharSequence, EncodingCapa
             }
             cp = spBeg + offset;
             if (offset > spLen) break;
-            beg = StringSupport.index(this, pattern, offset, patternEnc);
+            beg = StringSupport.index(getByteList(), pattern.getByteList(), offset, patternEnc);
         } while (beg >= 0);
 
         if (spLen > offset) dest.cat(spBytes, cp, spLen - offset, str_enc);
diff --git a/core/src/main/java/org/jruby/util/StringSupport.java b/core/src/main/java/org/jruby/util/StringSupport.java
index 5790ac76bb..6ee1c5d759 100644
--- a/core/src/main/java/org/jruby/util/StringSupport.java
+++ b/core/src/main/java/org/jruby/util/StringSupport.java
@@ -1616,6 +1616,38 @@ public final class StringSupport {
         return string.getCodeRange() == CR_7BIT || encoding.maxLength() == 1;
     }
 
+    /**
+     *
+     * @param source string to find index within
+     * @param other string to match in source
+     * @param offset in bytes to start looking
+     * @param enc encoding to use to walk the source string.
+     * @return
+     */
+    public static int index(ByteList source, ByteList other, int offset, Encoding enc) {
+        int sourceLen = source.realSize();
+        int sourceBegin = source.begin();
+        int otherLen = other.realSize();
+
+        if (otherLen == 0) return offset;
+        if (sourceLen - offset < otherLen) return -1;
+
+        byte[] sourceBytes = source.getUnsafeBytes();
+        int p = sourceBegin + offset;
+        int end = p + sourceLen;
+
+        while (true) {
+            int pos = source.indexOf(other, p - sourceBegin);
+            if (pos < 0) return pos;
+            pos -= (p - sourceBegin);
+            int t = enc.rightAdjustCharHead(sourceBytes, p, p + pos, end);
+            if (t == p + pos) return pos + offset;
+            if ((sourceLen -= t - p) <= 0) return -1;
+            offset += t - p;
+            p = t;
+        }
+    }
+
     public static int index(CodeRangeable sourceString, CodeRangeable otherString, int offset, Encoding enc) {
         if (otherString.scanForCodeRange() == CR_BROKEN) return -1;