code: ///////////////////////////////////////////////////////////////////////////////////////////////
  // checkstyle: Checks Java source code and other text files for adherence to a set of rules.
  // Copyright (C) 2001-2023 the original author or authors.
  //
  // This library is free software; you can redistribute it and/or
  // modify it under the terms of the GNU Lesser General Public
  // License as published by the Free Software Foundation; either
  // version 2.1 of the License, or (at your option) any later version.
  //
  // This library is distributed in the hope that it will be useful,
  // but WITHOUT ANY WARRANTY; without even the implied warranty of
  // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  // Lesser General Public License for more details.
  //
  // You should have received a copy of the GNU Lesser General Public
  // License along with this library; if not, write to the Free Software
  // Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  ///////////////////////////////////////////////////////////////////////////////////////////////
  
  package com.puppycrawl.tools.checkstyle.checks.naming;
  
  import java.util.regex.Pattern;
  
  import com.puppycrawl.tools.checkstyle.StatelessCheck;
  import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
  import com.puppycrawl.tools.checkstyle.api.DetailAST;
  import com.puppycrawl.tools.checkstyle.api.FullIdent;
  import com.puppycrawl.tools.checkstyle.api.TokenTypes;
  
  /**
   * <p>
   * Checks that package names conform to a specified pattern.
   * </p>
   * <p>
   * The default value of {@code format} for module {@code PackageName} has been chosen to match
   * the requirements in the
   * <a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-6.html#jls-6.5.3">
   * Java Language specification</a>
   * and the Sun coding conventions. However, both underscores and uppercase letters are rather
   * uncommon, so most configurations should probably assign value
   * {@code ^[a-z]+(\.[a-z][a-z0-9]*)*$} to {@code format} for module {@code PackageName}.
   * </p>
   * <ul>
   * <li>
   * Property {@code format} - Specifies valid identifiers.
   * Type is {@code java.util.regex.Pattern}.
   * Default value is {@code "^[a-z]+(\.[a-zA-Z_]\w*)*$"}.
   * </li>
   * </ul>
   * <p>
   * To configure the check:
   * </p>
   * <pre>
   * &lt;module name="PackageName"/&gt;
   * </pre>
   * <p>Code Example:</p>
   * <pre>
   * package com; // OK
   * package COM; // violation, name 'COM' must match pattern '^[a-z]+(\.[a-zA-Z_]\w*)*$'
   * package com.checkstyle.checks; // OK
   * package com.A.checkstyle.checks; // OK
   * package com.checkstyle1.checks; // OK
   * package com.checkSTYLE.checks; // OK
   * package com._checkstyle.checks_; // OK
   * </pre>
   * <p>
   * An example of how to configure the check to ensure with packages start with a lowercase letter
   * and only contains lowercase letters or numbers is:
   * </p>
   * <pre>
   * &lt;module name=&quot;PackageName&quot;&gt;
   *   &lt;property name=&quot;format&quot;
   *     value=&quot;^[a-z]+(\.[a-z][a-z0-9]*)*$&quot;/&gt;
   * &lt;/module&gt;
   * </pre>
   * <p>Code Example:</p>
   * <pre>
   * package com; // OK
   * package COM; // violation, name 'COM' must match pattern '^[a-z]+(\.[a-z][a-z0-9]*)*$'
   * package com.checkstyle.checks; // OK
   * package com.A.checkstyle.checks; // violation, name 'com.A.checkstyle' must match
   *                                  // pattern '^[a-z]+(\.[a-z][a-z0-9]*)*$'
   * package com.checkstyle1.checks; // OK
   * package com.checkSTYLE.checks; // violation, name 'com.checkSTYLE.checks' must
   *                                // match pattern '^[a-z]+(\.[a-z][a-z0-9]*)*$'
   * package com._checkstyle.checks_; // violation, name 'com._checkstyle.checks_' must match
   *                                  // pattern '^[a-z]+(\.[a-z][a-z0-9]*)*$'
   * </pre>
   * <p>
   * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
   * </p>
   * <p>
   * Violation Message Keys:
   * </p>
   * <ul>
   * <li>
   * {@code name.invalidPattern}
   * </li>
   * </ul>
   *
   * @since 3.0
   */
  @StatelessCheck
  public class PackageNameCheck
      extends AbstractCheck {
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_KEY = "name.invalidPattern";
  
      /** Specifies valid identifiers. */
      // Uppercase letters seem rather uncommon, but they're allowed in
      // https://docs.oracle.com/javase/specs/
      //  second_edition/html/packages.doc.html#40169
      private Pattern format = Pattern.compile("^[a-z]+(\\.[a-zA-Z_]\\w*)*$");
  
      /**
       * Setter to specifies valid identifiers.
       *
       * @param pattern the new pattern
       */
      public void setFormat(Pattern pattern) {
          format = pattern;
      }
  
      @Override
      public int[] getDefaultTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getAcceptableTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getRequiredTokens() {
          return new int[] {TokenTypes.PACKAGE_DEF};
      }
  
      @Override
      public void visitToken(DetailAST ast) {
          final DetailAST nameAST = ast.getLastChild().getPreviousSibling();
          final FullIdent full = FullIdent.createFullIdent(nameAST);
          if (!format.matcher(full.getText()).find()) {
              log(full.getDetailAst(),
                  MSG_KEY,
                  full.getText(),
                  format.pattern());
          }
      }
  
  }
  