Replacing occurences of a specific word if not proceeded by another specific word?

Question

I have some text like:

*open* blah blah blah blah blah *close* blah blah *open* blah blah *close* blah blah *close*

I was wondering how would I remove/replace any occurrences of *close* which are not proceeded by *open*.

So the above text would become:

*open* blah blah blah blah blah *close* blah blah *open* blah blah *close* blah blah

I though of maybe using regex with preg_replace - but my regex skills are not that strong?

<?php
$string = "*open* blah blah blah blah blah *close* blah blah *open* blah blah *close* blah blah *close*";

$string = preg_replace('#(?<!\*open\*)\*close\*#', '', $string); //this only works for immediate proceedings

echo($string);
?>

Code examples would be welcome.

Answer 1

It can be done without regex via the following code:

$openTag = '*open*';
$closeTag = '*close*';
$openTagLength = mb_strlen($openTag);
$closeTagLength = mb_strlen($closeTag);

$subj = '*open* blah blah blah blah blah *close* blah blah *open* blah blah *close* blah blah *close*';
$len = mb_strlen($subj);
$isOpened = false;
$res = '';
for ($i = 0; $i < $len; )
{
    if (mb_substr($subj, $i, $openTagLength) === $openTag) {
        // found open tag
        $res .= $openTag;
        $isOpened = true;
        $i += $openTagLength;
    } elseif (mb_substr($subj, $i, $closeTagLength) === $closeTag) {
        // found close tag
        if ($isOpened) {
            $res .= $closeTag;
        } // else skip
        $isOpened = false;
        $i += $closeTagLength;
    } else {
        // non-tag
        $res .= mb_substr($subj, $i, 1);
        $i++;
    }
}
echo $res;