Cancelling a long running regex match?

Question

Say I\'m running a service where users can submit a regex to search through lots of data. If the user submits a regex that is very slow (ie. takes minutes for Matcher.find()

Answer 1

A long-running pattern matching process can be stopped using the below method.

• Create StateFulCharSequence class which manages the state of pattern matching. When that state is changed, an exception is thrown on the next call to charAt method.
• That state change can be scheduled using ScheduledExecutorService with a required timeout.

• Here pattern matching is happening in the main thread and there is no need to check the thread interrupt state every time.

  public class TimedPatternMatcher {
  public static void main(String[] args) {
      ScheduledExecutorService executorService = Executors.newScheduledThreadPool(1);
      Pattern pattern = Pattern.compile("some regex pattern");
      StateFulCharSequence stateFulCharSequence = new StateFulCharSequence("some character sequence");
      Matcher matcher = pattern.matcher(stateFulCharSequence);
      executorService.schedule(stateFulCharSequence, 10, TimeUnit.MILLISECONDS);
      try {
          boolean isMatched = matcher.find();
      }catch (Exception e) {
          e.printStackTrace();
      }
  
  }
  
  /*
  When this runnable is executed, it will set timeOut to true and pattern matching is stopped by throwing exception.
   */
  public static class StateFulCharSequence implements CharSequence, Runnable{
      private CharSequence inner;
  
      private boolean isTimedOut = false;
  
      public StateFulCharSequence(CharSequence inner) {
          super();
          this.inner = inner;
      }
  
      public char charAt(int index) {
          if (isTimedOut) {
              throw new RuntimeException(new TimeoutException("Pattern matching timeout occurs"));
          }
          return inner.charAt(index);
      }
  
      @Override
      public int length() {
          return inner.length();
      }
  
      @Override
      public CharSequence subSequence(int start, int end) {
          return new StateFulCharSequence(inner.subSequence(start, end));
      }
  
      @Override
      public String toString() {
          return inner.toString();
      }
  
      public void setTimedOut() {
          this.isTimedOut = true;
      }
  
      @Override
      public void run() {
          this.isTimedOut = true;
      }
  }}