code: /*
  IllegalIdentifierName
  format = (default)(?i)^(?!(record|yield|var|permits|sealed|_)$).+$
  tokens = (default)CLASS_DEF, INTERFACE_DEF, ENUM_DEF, ANNOTATION_DEF, ANNOTATION_FIELD_DEF, \
           PARAMETER_DEF, VARIABLE_DEF, METHOD_DEF, ENUM_CONSTANT_DEF, PATTERN_VARIABLE_DEF, \
           RECORD_DEF, RECORD_COMPONENT_DEF, LAMBDA
  
  
  */
  
  //non-compiled with javac: compiling on jdk before 9
  package com.puppycrawl.tools.checkstyle.checks.naming.illegalidentifiername;
  
  public class InputIllegalIdentifierNameUnderscore {
      String _string = "_string"; // ok
      String string_ = "string_"; // ok
      String another_string = "string"; // ok
      String _; // violation
  }
  