[SPARK-16409][SQL] regexp_extract with optional groups causes NPE

## What changes were proposed in this pull request?

regexp_extract actually returns null when it shouldn't when a regex matches but the requested optional group did not. This makes it return an empty string, as apparently designed.

## How was this patch tested?

Additional unit test

Author: Sean Owen <sowen@cloudera.com>

Closes #14504 from srowen/SPARK-16409.

1 files changed, 8 insertions, 0 deletions

diff --git a/sql/core/src/test/scala/org/apache/spark/sql/StringFunctionsSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/StringFunctionsSuite.scala
index 57ca5d9c4d..3b76aaf7d0 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/StringFunctionsSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/StringFunctionsSuite.scala
@@ -94,6 +94,14 @@ class StringFunctionsSuite extends QueryTest with SharedSQLContext {
       Row("300", "100") :: Row("400", "100") :: Row("400-400", "100") :: Nil)
   }
 
+  test("non-matching optional group") {
+    val df = Seq("aaaac").toDF("s")
+    checkAnswer(
+      df.select(regexp_extract($"s", "(a+)(b)?(c)", 2)),
+      Row("")
+    )
+  }
+
   test("string ascii function") {
     val df = Seq(("abc", "")).toDF("a", "b")
     checkAnswer(