Does this PHP code crash Apache for anyone else?

Question

preg_match_all('/(a)*/', str_repeat('a', 1000), $matches);

(edit: change the regexp a bit to make it simpler while still crashing)

I ran it on PHP 5.3.5 with Apache 2.0 and it crashes the server. If I change 339 to 338 it doesn't crash anymore, so it seems like a bug to me. I tried reporting it to http://bugs.php.net/ but it's down. Is this a PHP bug? Does it crash for anyone else?

Edit: Changing the code to

preg_match_all('/(?:a)*/', str_repeat('a', 339), $matches);

allows for a longer string before crashing. If it doesn't crash, try increasing the string length by a factor of 10 or 100 as it may be a memory issue and you may have more memory.

Edit 2: the crash is a complete process crash, on Windows 7 I get the "End task" message instantaneously after execution.

Edit 3: if the crash is due to too much backtracing, and the above example clearly can cause it, the following should not:

preg_match('/[^"\']*(;|$)/',
               str_repeat('x', 1000), $matches);

This is my actual code that's crashing. It's simply meant to split multiple SQL queries by ;, while allowing ; inside single or double quotes. Why is this causing so much backtracing, and how can I fix it?

ini_set('max_execution_time', 1500); ini_set("memory_limit", "32M"); — Sahal
– Sahal, Commented Jun 17, 2011 at 6:56
Ah, Windows. My guess would be something wrong with your build/it's PCRE library. Definitely local to your config. Also, not sure why you are editing the crash condition or telling us to run it 100 times longer -- ideally you want to isolate the simplest/shortest thing that makes it crash and trace from there. — joelhardi
– joelhardi, Commented Jun 17, 2011 at 7:19

Francois Deschenes · Accepted Answer · 2011-06-17 07:16:27Z

4

The problem isn't memory or execution time...

ini_set('pcre.backtrack_limit', 10000);

Feel free to decrease the 10000 to a different value if necessary. For more information, see http://php.net/manual/en/pcre.configuration.php.

If you feel like testing where it crashes:

<?php

ini_set('pcre.backtrack_limit', 10000);

for ( $i = 1; $i < 65535; $i++ )
{
  echo $i . PHP_EOL;
  preg_match_all('/(a)*/', str_repeat('a', $i), $matches);
}

?>

edited Jun 17, 2011 at 7:16

answered Jun 17, 2011 at 7:10

Francois Deschenes

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7 Comments

manixrock Over a year ago

I tried it with 10 000, 100 000 and 1 000 000 all with the same result of instantaneous crash. But why is it backtracking? Shouldn't /(a)*/ greedily match all the text, and return really fast?

Francois Deschenes Over a year ago

@manixrock - You can read about it here: http://www.regular-expressions.info/catastrophic.html.

manixrock Over a year ago

Ok, decreasing it makes more sense. With 500 it doesn't crash for 1000-chars string. But I still don't understand why it's backtracking. The match is greedy, and it should match all on the first run, then stop.

Francois Deschenes Over a year ago

@manixrock - Read the article I linked to above. It explains what backtracking is. It should make sense when you're done reading it.

manixrock Over a year ago

@Francois Deschenes - I knew about that problem with regexp, but I thought preg_match_all() wouldn't backtrack between the different match parts. But this does not explain why my main query, which shouldn't allow for backtracing, still crashes. I'll edit my main post in a minute...

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