<?php
$input = <<<EOD
<p>sapien Cras cursus consequat nibh <a href="#">sapien</a>ac vehicula. Sed erat sapien, condimentum quis risus nec, viverra dignissim nisi. Cras sapien convallis, erat egestas tincidunt <img src="myimage.jpg" alt="sapien" >rutrum, massa enim sagittis ante, sapien.sed pellentesque lorem risus vitae enim. Curabitur hendrerit dolor facilisis <a href="sapien">sapien</a> dolor malesuada molestie.sapien</p>
EOD;
 
// define tags which you need to exclude from replacement 
// as: array( start_string => end_string, ... );
$ignore_tags = array(
    '<a' => '</a>',
    '<img' => '>',
    '<div' => '</div>'
);
 
echo "Input:\n {$input} \n\n ";
$output = parse_text($input, $ignore_tags);
echo "Output:\n {$output}";
 
// callback function that invokes every time when 'parse_text' parses 'clean' peace of text
function my_replace($text) {
	echo "my_replace call on: \n".$text."\n\n";
 
    // your replacements here
    $text = preg_replace("#\b(sapien)\b#i", "<a href=#>\\1</a>", $text);
    return $text;
}
 
 
// main parsing function that split text to clean and ignored parts
function parse_text($input, $ignore_tags) {
    $output = '';
    $str = '';
    $ignore = false;
    $current_ignore_tag = '';
    $ignore_tags_regexp = implode("|", array_keys($ignore_tags));
 
    for ($i = 0; $i < strlen($input); $i++) {
        $str .= $input[$i];
        // if starts ignore tag and not already $ignore
        if (preg_match("#({$ignore_tags_regexp})$#si", $str, $m) && !$ignore) {
            $str = preg_replace("#({$ignore_tags_regexp})$#si", "", $str); // cut and not include ignore tag
            $output .= my_replace($str) . $m[1]; // replace all before and save
            $ignore = true;
            $current_ignore_tag = $m[1];
            $str = '';
        } // if $ignore and matches the end of the current ignore tag
        elseif ($ignore && preg_match("#({$ignore_tags[$current_ignore_tag]})$#i", $str, $m)) {
            $output .= $str; // just save as it is current peace
            $ignore = false;
            $str = '';
        }
    }
    $output .= (!$ignore) ? my_replace($str) : $str;
    return $output;
}
 
 

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