Regular Expression Help - Brackets within brackets

Question

Hello

I'm trying to develop a function that can sort through a string that looks like this:

Donny went to the {park|store|{beach with friends|beach alone}} so he could get a breath of fresh air. 

What I intend to do is search the text recursively for {} patterns where there is no { or } inside the {}, so only the innermost sandwiched text is selected, where I will then run a php to array the contents and select one at random, repeating process until the whole string has been parsed, showing a complete sentence.

I just cannot wrap my head around regular expressions though.

Appreciate any help!

Answer 1

Regexes are not capable of counting and therefore cannot find matching brackets reliably.

What you need is a grammar.

See this related question.

Answer 2

You could do this with a lexer/parser. I don't know of any options in PHP (but since there are XML parsers in PHP, there are no doubt generic parsers). On the other hand, what you're asking to do is not too complicated. Using strings in PHP (substring, etc.) you could probably do this in a few recursive functions.

You will then finally have created a MadLibz generator in PHP with a simple grammar. Pretty cool.

Answer 3

$str="Donny went to the {park|store|{beach {with friends}|beach alone}} so he could get a breath of fresh air. ";
$s = explode("}",$str);
foreach($s as $v){
 if(strpos($v,"{")!==FALSE){
  $t=explode("{",$v);
  print end($t)."\n";
 }
}

output

$ php test.php
with friends

Answer 4

Don't know about maths theory behind this ;-/ but in practice that's quite easy. Try

$text = "Donny went to the {park|store|{beach with friends|beach alone}} so he could get a breath of fresh air. ";

function rnd($matches) {
    $words = explode('|', $matches[1]);
    return $words[rand() % count($words)];
}

do {
    $text = preg_replace_callback('~{([^{}]+)}~', 'rnd', $text, -1, $count);
} while($count > 0);

echo $text;

Answer 5

Regular expressions don't deal well with recursive stuff, but PHP does:

$str = 'Donny went to the {park|store|{beach with friends|beach alone}} so he could get a breath of fresh air.';

echo parse_string($str), "\n";

function parse_string($string) {
    if ( preg_match('/\{([^{}]+)\}/', $string, $matches) ) {
        $inner_elements = explode('|', $matches[1]);
        $random_element = $inner_elements[array_rand($inner_elements)];
        $string = str_replace($matches[0], $random_element, $string);
        $string = parse_string($string);
    }
    return $string;
}