PHP preg_replace crashes. Only for Regexp Masters

Question

How are you? I'll get straight to the point.

I'm using a recursive regular expression that basically removes individual or nested <blockquote> tags. I only need to remove plain <blockquote> ... </blockquote> text, nested or not, and leave whatever is outside of these.

This regex does the job EXACTLY as I want (note the use of lookahead and recursion)

$comment=preg_replace('#<blockquote>((?!(</?blockquote>)).|(?R))*</blockquote>#s',"",$comment);

but it has a big problem: when the $comment is large (more than 3500 characters long), apache crashes (I assume segmentation fault).

I need a solution to the problem, either but solving the crash, using a better regexp or a custom function that will do the job as well.

If you simply have ideas on how to remove nested specific tags, they are kindly welcome.

Thank you in advance

Please refrain from parsing HTML with RegEx as it will [drive you į̷̷͚̤̤̖̱̦͍͗̒̈̅̄̎n̨͖͓̹͍͎͔͈̝̲͐ͪ͛̃̄͛ṣ̷̵̞̦ͤ̅̉̋ͪ͑͛ͥ͜a̷̘͖̮͔͎͛̇̏̒͆̆͘n͇͔̤̼͙̩͖̭ͤ͋̉͌͟eͥ͒͆ͧͨ̽͞҉̹͍̳̻͢](http://stackoverflow.com/questions/1732348/regex-match-open-tags-except-xhtml-self-contained-tags/1732454#1732454). Use an [HTML parser](http://stackoverflow.com/questions/292926/robust-mature-html-parser-for-php) instead. — Madara's Ghost, Aug 10 '12 at 23:29
Segfault = [stack overflow](http://en.wikipedia.org/wiki/Stack_overflow) = probably infinite recursion. Are you sure it is simply the size of the string, or is it the content of it that is causing the problem? Although @Truth speaks the *truth* (ha!) - an HTML parser is a much better tool for this job. If you must persist with regex and you are certain the expression suits your needs, try throwing an `S` (study) flag on it, I have seen it fix a multitude of sins. — DaveRandom, Aug 11 '12 at 00:00
@Truth Congrats, you seem to have found a way to break to SO CSS with that comment. Skillz... — DaveRandom, Aug 11 '12 at 00:01
@Dandy [Here](http://codepad.viper-7.com/2lFcsr) is an example of using an HTML parser to strip blockquotes from your string. — DaveRandom, Aug 11 '12 at 00:19
Thank you all. You were all helpful, and I went for the @cleong solution. I'm really grateful. — Dandy, Aug 11 '12 at 02:11

score 1 · Accepted Answer · answered Aug 11 '12 at 00:38

Man, your pattern sigfaults like crazy! Even comment of several hundred bytes ends with a crash.

It's a lot simpler to use preg_split() to split up the string, then use a counter to keep track of how deep you are. And when the depth is greater than one, you throw away the text. Here's the implementation:

$tokens = preg_split('#(</?blockquote.*?>)#s', $comment, -1, PREG_SPLIT_DELIM_CAPTURE); 
$outsideTokens = array();
$depth = 0;
for($token = reset($tokens); $token !== false; $token = next($tokens)) { 
    if($depth == 0) {
        $outsideTokens[] = $token;
    }
    $delimiter = next($tokens);
    if($delimiter[1] == '/') {
        $depth--;
    } else {
        $depth++;
    }
}
$comment = implode($outsideTokens);

The code should work even when the start tag contains attributes.

Wow! You just did it. I could not figure it out and you just did it! Congratulations and many many thanks! — Dandy, Aug 11 '12 at 02:00

PHP preg_replace crashes. Only for Regexp Masters

1 Answers1