Fix EmptyCharSequence returning 'INSTANCE' as it's toString representation

src/main/java/com/github/fge/largetext/sequence/EmptyCharSequence.java

@@ -51,4 +51,9 @@ public CharSequence subSequence(final int start, final int end)
         return this;
     }
 
+    @Override
+    public String toString()
+    {
+        return "";
+    }
 }

src/test/java/com/github/fge/largetext/sequence/EmptyCharSequenceTest.java

@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2003 - 2017 Tyro Payments Limited.
+ * Lv1, 155 Clarence St, Sydney NSW 2000.
+ * All rights reserved.
+ */
+package com.github.fge.largetext.sequence;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.io.File;
+import java.io.IOException;
+import java.io.PrintWriter;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+import org.testng.annotations.Test;
+
+import com.github.fge.largetext.LargeText;
+import com.github.fge.largetext.LargeTextFactory;
+
+public final class EmptyCharSequenceTest {
+
+    @Test
+    public void testEmptyRegexMatchReturnsAnEmptyString() throws IOException
+    {
+        Pattern simpleEmptyCharsetPattern = Pattern.compile("'(.*?)'");
+        String stringToTest = "here is some text '' will be an empty regex match";
+        File temp = File.createTempFile("file-with-contents-that-match-empty-charset-regex", ".tmp");
+        try (PrintWriter out = new PrintWriter(temp))
+        {
+            out.println(stringToTest);
+        }
+
+        final LargeTextFactory factory = LargeTextFactory.defaultFactory();
+
+        try (LargeText largeText = factory.load(temp.toPath()))
+        {
+            final Matcher largeTextMatcher = simpleEmptyCharsetPattern.matcher(largeText);
+            while (largeTextMatcher.find())
+            {
+                String emptyRegexMatch = largeTextMatcher.group(1);
+                System.out.println("emptyRegexMatch = " + emptyRegexMatch);
+                assertThat(emptyRegexMatch).isEqualTo("").overridingErrorMessage("LargeText did not return empty string: %s", emptyRegexMatch);
+            }
+        }
+
+        temp.delete();
+    }
+}