Split string by delimiter, but not if it is escaped

How can I split a string by a delimiter, but not if it is escaped? For example, I have a string:

1|2\|2|3\\|4\\\|4

The delimiter is | and an escaped delimiter is \|. Furthermore I want to ignore escaped backslashes, so in \\| the | would still be a delimiter.

So with the above string the result should be:

[0] => 1
[1] => 2\|2
[2] => 3\\
[3] => 4\\\|4

Use dark magic:

$array = preg_split('~\\\\.(*SKIP)(*FAIL)|\|~s', $string);

\\\\. matches a backslash followed by a character, (*SKIP)(*FAIL) skips it and \| matches your delimiter.

Instead of split(...), it's IMO more intuitive to use some sort of "scan" function that operates like a lexical tokenizer. In PHP that would be the preg_match_all function. You simply say you want to match:

1. something other than a \ or |
2. or a \ followed by a \ or |
3. repeat #1 or #2 at least once

The following demo:

$input = "1|2\\|2|3\\\\|4\\\\\\|4";
echo $input . "\n\n";
preg_match_all('/(?:\\\\.|[^\\\\|])+/', $input, $parts);
print_r($parts[0]);

will print:

1|2\|2|3\\|4\\\|4

Array
(
    [0] => 1
    [1] => 2\|2
    [2] => 3\\
    [3] => 4\\\|4
)

Recently I devised a solution:

$array = preg_split('~ ((?<!\\\\)|(?<=[^\\\\](\\\\\\\\)+)) \| ~x', $string);

But the black magic solution is still three times faster.