[ASTERIXDB-2247][SQLPP] Allow "_" as the first character in a variable name

- user model changes: yes
- storage format changes: no
- interface changes: no

Details:
- Allow "_" as the first character in a variable name and
  other identifiers

Change-Id: Ic530ccfd2a7d3f1e893fd2292b8605feb6241675
Reviewed-on: https://asterix-gerrit.ics.uci.edu/2283
Sonar-Qube: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Reviewed-by: Michael Blow <mblow@apache.org>
Tested-by: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Contrib: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Integration-Tests: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
diff --git a/asterixdb/asterix-app/src/test/resources/parserts/queries_sqlpp/variables.sqlpp b/asterixdb/asterix-app/src/test/resources/parserts/queries_sqlpp/variables.sqlpp
index 7611e3e..417bea8 100644
--- a/asterixdb/asterix-app/src/test/resources/parserts/queries_sqlpp/variables.sqlpp
+++ b/asterixdb/asterix-app/src/test/resources/parserts/queries_sqlpp/variables.sqlpp
@@ -18,6 +18,7 @@
  */
 
 with  a as 1,
-      b as 1
-select element (b - a)
+      b as 1,
+      _c as 0
+select element (b - a - _c)
 ;
diff --git a/asterixdb/asterix-app/src/test/resources/parserts/results_parser_sqlpp/variables.ast b/asterixdb/asterix-app/src/test/resources/parserts/results_parser_sqlpp/variables.ast
index 698f469..9f6906c 100644
--- a/asterixdb/asterix-app/src/test/resources/parserts/results_parser_sqlpp/variables.ast
+++ b/asterixdb/asterix-app/src/test/resources/parserts/results_parser_sqlpp/variables.ast
@@ -5,10 +5,15 @@
 Let Variable [ Name=$b ]
   :=
   LiteralExpr [LONG] [1]
+Let Variable [ Name=$_c ]
+  :=
+  LiteralExpr [LONG] [0]
 SELECT ELEMENT [
 OperatorExpr [
   Variable [ Name=$b ]
   -
   Variable [ Name=$a ]
+  -
+  Variable [ Name=$_c ]
 ]
 ]
diff --git a/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/misc/identifier_01/identifier_01.1.query.sqlpp b/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/misc/identifier_01/identifier_01.1.query.sqlpp
new file mode 100644
index 0000000..17ad534
--- /dev/null
+++ b/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/misc/identifier_01/identifier_01.1.query.sqlpp
@@ -0,0 +1,22 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+WITH _x as 1
+
+SELECT _x AS _y
diff --git a/asterixdb/asterix-app/src/test/resources/runtimets/results/misc/identifier_01/identifier_01.1.adm b/asterixdb/asterix-app/src/test/resources/runtimets/results/misc/identifier_01/identifier_01.1.adm
new file mode 100644
index 0000000..864e169
--- /dev/null
+++ b/asterixdb/asterix-app/src/test/resources/runtimets/results/misc/identifier_01/identifier_01.1.adm
@@ -0,0 +1 @@
+{ "_y": 1 }
diff --git a/asterixdb/asterix-app/src/test/resources/runtimets/testsuite_sqlpp.xml b/asterixdb/asterix-app/src/test/resources/runtimets/testsuite_sqlpp.xml
index 14784d6..0b8fd8c 100644
--- a/asterixdb/asterix-app/src/test/resources/runtimets/testsuite_sqlpp.xml
+++ b/asterixdb/asterix-app/src/test/resources/runtimets/testsuite_sqlpp.xml
@@ -3518,6 +3518,11 @@
       </compilation-unit>
     </test-case>
     <test-case FilePath="misc">
+      <compilation-unit name="identifier_01">
+        <output-dir compare="Text">identifier_01</output-dir>
+      </compilation-unit>
+    </test-case>
+    <test-case FilePath="misc">
       <compilation-unit name="ifthenelse_01">
         <output-dir compare="Text">ifthenelse_01</output-dir>
       </compilation-unit>
diff --git a/asterixdb/asterix-lang-sqlpp/src/main/javacc/SQLPP.jj b/asterixdb/asterix-lang-sqlpp/src/main/javacc/SQLPP.jj
index a11aaf4..fd5de86 100644
--- a/asterixdb/asterix-lang-sqlpp/src/main/javacc/SQLPP.jj
+++ b/asterixdb/asterix-lang-sqlpp/src/main/javacc/SQLPP.jj
@@ -3399,7 +3399,8 @@
 TOKEN :
 {
     <#LETTER : ["A" - "Z", "a" - "z"]>
-  | <SPECIALCHARS : ["$", "_"]>
+  | <#IDENTIFIER_SPECIALCHARS_START : ["_"]>
+  | <#IDENTIFIER_SPECIALCHARS_REST : ["$"]>
 }
 
 <DEFAULT,IN_DBL_BRACE>
@@ -3450,7 +3451,8 @@
 <DEFAULT,IN_DBL_BRACE>
 TOKEN :
 {
-    <IDENTIFIER : <LETTER> (<LETTER> | <DIGIT> | <SPECIALCHARS>)*>
+    <IDENTIFIER : ( <LETTER> | <IDENTIFIER_SPECIALCHARS_START> )
+                  ( <LETTER> | <DIGIT> | <IDENTIFIER_SPECIALCHARS_START> | <IDENTIFIER_SPECIALCHARS_REST> )*>
 }
 
 <DEFAULT,IN_DBL_BRACE>