[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, 11 insertions, 2 deletions

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/regexpExpressions.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/regexpExpressions.scala
index be82b3b8f4..d25da3fd58 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/regexpExpressions.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/regexpExpressions.scala
@@ -329,7 +329,12 @@ case class RegExpExtract(subject: Expression, regexp: Expression, idx: Expressio
     val m = pattern.matcher(s.toString)
     if (m.find) {
       val mr: MatchResult = m.toMatchResult
-      UTF8String.fromString(mr.group(r.asInstanceOf[Int]))
+      val group = mr.group(r.asInstanceOf[Int])
+      if (group == null) { // Pattern matched, but not optional group
+        UTF8String.EMPTY_UTF8
+      } else {
+        UTF8String.fromString(group)
+      }
     } else {
       UTF8String.EMPTY_UTF8
     }
@@ -367,7 +372,11 @@ case class RegExpExtract(subject: Expression, regexp: Expression, idx: Expressio
         ${termPattern}.matcher($subject.toString());
       if (${matcher}.find()) {
         java.util.regex.MatchResult ${matchResult} = ${matcher}.toMatchResult();
-        ${ev.value} = UTF8String.fromString(${matchResult}.group($idx));
+        if (${matchResult}.group($idx) == null) {
+          ${ev.value} = UTF8String.EMPTY_UTF8;
+        } else {
+          ${ev.value} = UTF8String.fromString(${matchResult}.group($idx));
+        }
         $setEvNotNull
       } else {
         ${ev.value} = UTF8String.EMPTY_UTF8;