code: package it.unibz.inf.ontop.spec.sqlparser;
  
  /*
   * #%L
   * ontop-obdalib-core
   * %%
   * Copyright (C) 2009 - 2014 Free University of Bozen-Bolzano
   * %%
   * Licensed 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.
   * #L%
   */
  
  import com.google.common.collect.ImmutableList;
  import it.unibz.inf.ontop.dbschema.*;
  import it.unibz.inf.ontop.exception.InvalidQueryException;
  
  import java.util.regex.Matcher;
  import java.util.regex.Pattern;
  
  
  /**
   * Created by Roman Kontchakov on 09/01/2017.
   */
  
  public class ApproximateSelectQueryAttributeExtractor {
  
      private final QuotedIDFactory idfac;
  
      private static final Pattern AS = Pattern.compile("\\sAS\\s", Pattern.CASE_INSENSITIVE);
      private static final Pattern BRACKETS = Pattern.compile("\\([^()]*\\)");
      private static final Pattern COL_SEP = Pattern.compile(",");
      private static final Pattern SELECT = Pattern.compile("\\A[\\s]*SELECT\\s+(DISTINCT\\s)?", Pattern.CASE_INSENSITIVE);
      private static final Pattern FROM = Pattern.compile("\\sFROM\\s", Pattern.CASE_INSENSITIVE);
  
      public ApproximateSelectQueryAttributeExtractor(QuotedIDFactory idfac) {
          this.idfac = idfac;
      }
  
      public ImmutableList<QuotedID> getAttributes(String sql) throws InvalidQueryException {
  
          // COULD NOT PARSE - do a rough approximation
  
          Matcher startMatcher = SELECT.matcher(sql);
          if (!startMatcher.find())
              throw new InvalidQueryException("Error parsing SQL query: Couldn't find SELECT clause", sql);
          int start = startMatcher.end();
  
          Matcher endMatcher = FROM.matcher(sql);
          if (!endMatcher.find())
              throw new InvalidQueryException("Error parsing SQL query: Couldn't find FROM clause", sql);
          int end = endMatcher.start();
  
          String projection = sql.substring(start, end);
  
          // remove all brackets
          for (Matcher matcher = BRACKETS.matcher(projection); matcher.find(); matcher = BRACKETS.matcher(projection))
              projection = matcher.replaceAll("");
  
          final ImmutableList.Builder<QuotedID> attributes = ImmutableList.builder();
          for (String column : COL_SEP.split(projection)) {
              String[] components = AS.split(column);
              // components = { column } if there is no AS
              String columnName = components[components.length - 1].trim();
  
              // ROMAN (25 Jan 2017): do not understand the purpose
              // split on spaces that are not inside single quotes
              // if (columnName.contains(" "))
              //    columnName = columnName.split("\\s+(?![^'\"]*')")[1].trim();
  
              // get only the column name (but not the qualifier table name)
              // eg: table.column -> column
              columnName = columnName.substring(columnName.lastIndexOf(".") + 1);
  
              QuotedID attribute = idfac.createAttributeID(columnName);
              attributes.add(attribute);
          }
  
          return attributes.build();
      }
  }
  