PHP preg_match length 3276 limit

Question

It appears that PHP's preg_match has a 3276 character limit for matching repeating characters in some cases.

i.e.

^(.|\s){0,3276}$ works, but ^(.|\s){0,3277}$ does not.

It doesn't seem to always apply, as /^(.){0,3277}$/ works.

I can't find this mentioned anywhere in PHP's documentation or the bug tracker. The number 3276 seems a bit of an odd boundary, the only thing I can think of is that it's approximately 1/10th of 32767, which is the limit for a signed 16-bit integer.

preg_last_error() returns 0.

I've reproduced the issue on http://www.phpliveregex.com/ as well as my local system and the webserver.

EDIT: Looks like we're getting "Warning: preg_match(): Compilation failed: regular expression is too large at offset 16" out of the code, so it appears to be the same issue as PHP preg_match_all limit.

However, the regex itself isn't very large... Does PHP do some kind of expansion when you have repeating groups that's making it too large?

Answer 1

In order to handle Perl-compatible regular expressions, PHP just bundles a third-party library that takes care of the job. The behaviour you describe is actually documented:

The "*" quantifier is equivalent to {0,} , the "+" quantifier to {1,} , and the "?" quantifier to {0,1} . n and m are limited to non-negative integral values less than a preset limit defined when perl is built. This is usually 32766 on the most common platforms.

So there's always a hard limit. Why do your tests suggest that PHP limit is 10 times smaller than the typical one? No idea about that :)