Description:
------------
Documentation of how PCRE regex behaves with escaped backslashes is confusing as written in current documentation:

http://php.planetmirror.com/manual/en/reference.pcre.pattern.syntax.php

Perhaps this just means an expansion of the note:

"Note:  Single and double quoted PHP strings have special meaning of backslash. Thus if \ has to be matched with a regular expression \\, then "\\\\" or '\\\\' must be used in PHP code."

While trying to use preg_split to break a string at non-escaped string quotes, I tried using a negative lookbehind assertion (see code below). With all other escaped characters in the lookbehind assertion, you only need one backwards slash before the escaped character (e.g., "(?<!\s)" works); but if you want to escape a backslash, you need to escape it twice (e.g., "(?<!\\\)" works but "(?<!\\)" does not). 

Based on the documentation's statement 'In particular, if you want to match a backslash, you write "\\".' it seems that (?<!\\) should work, and the extra information in the Note is too terse to explain why.

Perhaps this is actually a bug, but I can't tell if it's normal behavior that could be clarified more in documentation.

The closest I could find for bug reports to this issue is here:

http://bugs.php.net/bug.php?id=22315

...but this only implies the difference in behavior without explaining why.

Reproduce code:
---------------
$string = "This sentence is not quoted. 'But this one is, and it contains \'escaped quotes\' within!'";

$needle = "/(?<!\\)'/";
$results = preg_split($needle, $string);

print_r($results);

Expected result:
----------------
Array
(
    [0] => This sentence is not quoted. 
    [1] => But this one is, and it contains \'escaped quotes\' within!
    [2] => 
)

// Adding a third backslash to the $needle gives this result.

Actual result:
--------------
Warning: preg_split() [function.preg-split]: Compilation failed: missing ) at offset 7 in /data/domains/lawweb3.law.umn.edu/public/php/regex.php on line 16