Parsing a string with recursive parentheses

Question

I\'m trying to parse a string with the following structure in PHP:

a,b,c(d,e,f(g),h,i(j,k)),l,m,n(o),p

For example, a \"real\" string will

Answer 1

As Wiktor pointed out, this can be achieved with the help of a lexer. The following answer uses a class originally from Nikita Popopv, which can be found here.

What it does

It skims through the string and searches for matches as defined in the $tokenMap. These are defined as T_FIELD, T_SEPARATOR, T_OPEN and T_CLOSE. The values found are put in an array called $structure.
Afterwards we need to loop over this array and build the structure out of it. As there can be multiple nestings, I chose a recursive approach (generate()).

Demo

A demo can be found on ideone.com.

Code

The actual code with explanations:

// this is our $tokenMap
$tokenMap = array(
    '[^,()]+'       => T_FIELD,     # not comma or parentheses
    ','             => T_SEPARATOR, # a comma
    '\('            => T_OPEN,      # an opening parenthesis
    '\)'            => T_CLOSE      # a closing parenthesis
);

// this is your string
$string = "id,topic,member(name,email,group(id,name)),message(id,title,body)";

// a recursive function to actually build the structure
function generate($arr=array(), $idx=0) {
    $output = array();
    $current = null;
    for($i=$idx;$i<count($arr);$i++) {
        list($element, $type) = $arr[$i];
        if ($type == T_OPEN)
            $output[$current] = generate($arr, $i+1);
        elseif ($type == T_CLOSE)
            return $output;
        elseif ($type == T_FIELD) {
            $output[$element] = null;
            $current = $element;
        }
    }
    return $output;
}

$lex = new Lexer($tokenMap);
$structure = $lex->lex($string);

print_r(generate($structure));