Disallows empty strings that were allowed in regular expressions.(eclipse-ee4j#22848)

Signed-off-by: endtak <endo.takafumi@jp.fujitsu.com>
diff --git a/appserver/jdbc/jdbc-config/src/main/java/org/glassfish/jdbc/config/JdbcConnectionPool.java b/appserver/jdbc/jdbc-config/src/main/java/org/glassfish/jdbc/config/JdbcConnectionPool.java
index 4ce183f..fa7fa6e 100644
--- a/appserver/jdbc/jdbc-config/src/main/java/org/glassfish/jdbc/config/JdbcConnectionPool.java
+++ b/appserver/jdbc/jdbc-config/src/main/java/org/glassfish/jdbc/config/JdbcConnectionPool.java
@@ -273,7 +273,7 @@
      *         {@link String }
      */
     @Attribute
-    @Pattern(regexp="(read-uncommitted||read-committed|repeatable-read|serializable)")
+    @Pattern(regexp="(read-uncommitted|read-committed|repeatable-read|serializable)")
     String getTransactionIsolationLevel();
 
     /**