Question

I have a file that is structured in a large multidimensional structure, similar to json, but not close enough for me to use a json library.

The data looks something like this:

alpha {
    beta {
        charlie;
    }
    delta;
}

echo;
foxtrot {
    golf;
    hotel;
}

The regex I am trying to build (for a preg_match_all) should match each top level parent (delimited by {} braces) so that I can recurse through the matches, building up a multidimensional php array that represents the data.

The first regex I tried is /(?<=\{).*(?=\})/s which greedily matches content inside braces, however this isn’t quite right as when there is more than one sibling in the top level the match is too greedy. Example below:

Using regex /(?<=\{).*(?=\})/s match is given as:

Match 1:

    beta {
        charlie;
    }
    delta;
}

echo;
foxtrot {
    golf;
    hotel;

Instead the result should be:
Match 1:

    beta {
        charlie;
    }
    delta;

Match 2:

    golf;
    hotel;

So regex wizards, what function am I missing here or do I need to solve this with php somehow? Any tips very welcome 🙂

Answer 1

You can’t ¹ do this with regular expressions.

Alternatively, if you want to match deep-to-shallow blocks, you can use \{[^\{\}]*?\} and preg_replace_callback() to store the value, and return null to erase it from the string. The callback will need to take care of nesting the value accordingly.

$heirarchalStorage = ...;
do {
    $string = \preg_replace_callback('#\{[^\{\}]*?\}#', function($block)
    use(&$heirarchalStorage) {
        // do your magic with $heirarchalStorage
        // in here
        return null;
    }, $string);
} while (!empty($string));

_{Incomplete, not tested, and no warranty.}

This approach requires that the string be wrapped in {} as well, otherwise the final match won’t happen and you’ll loop forever.

This is an awful lot of (inefficient) work for something that can just as easily be solved with a well known exchange/storage format such as JSON.

¹ I was going to put “you can, but…“, however I’ll just say once again, “You can’t” ²

² Don’t