PHP Regex expression excluding <pre> tag

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I am using a WordPress plugin named Acronyms (https://wordpress.org/plugins/acronyms/). This plugin replaces acronyms with their description. It uses a PHP PREG_REPLACE function.

The issue is that it replaces the acronyms contained in a <pre> tag, which I use to present a source code.

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Could you modify this expression so that it won’t replace acronyms contained inside <pre> tags (not only directly, but in any moment)? Is it possible?

The PHP code is: 

$text = preg_replace(
    "|(?!<[^<>]*?)(?<![?.&])b$acronymb(?!:)(?![^<>]*?>)|msU"
  , "<acronym title="$fulltext">$acronym</acronym>"
  , $text
);
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2 comments

  1. You can use a PCRE SKIP/FAIL regex trick (also works in PHP) to tell the regex engine to only match something if it is not inside some delimiters:

    (?s)<pre[^<]*>.*?</pre>(*SKIP)(*F)|b$acronymb

    This means: skip all substrings starting with <pre> and ending with </pre>, and only then match $acronym as a whole word.

    See demo on regex101.com

    Here is a sample PHP demo:

    <?php
$acronym = "ASCII";
$fulltext = "American Standard Code for Information Interchange";
$re = "/(?s)<pre[^<]*>.*?<\/pre>(*SKIP)(*F)|\b$acronym\b/"; 
$str = "<pre>ASCIInSometextnMoretext</pre>More text nASCIInMore text<pre>MorenlinesnASCIInlines</pre>"; 
$subst = "<acronym title="$fulltext">$acronym</acronym>"; 
$result = preg_replace($re, $subst, $str);
echo $result;

    Output:

    <pre>ASCII</pre><acronym title="American Standard Code for Information Interchange">ASCII</acronym><pre>ASCII</pre>

  2. It is also possible to use preg_split and keep the code block as a group, only replace the non-code block part then combine it back as a complete string:

    function replace($s) {
    return str_replace('"', '&quot;', $s); // do something with `$s`
}

$text = 'Your text goes here...';
$parts = preg_split('#(</?[-:w]+(?:s[^<>]+?)?>)#', $text, null, PREG_SPLIT_NO_EMPTY | PREG_SPLIT_DELIM_CAPTURE);
$text = "";
$x = 0;
foreach ($parts as $v) {
    if (trim($v) === "") {
        $text .= $v;
        continue;
    }
    if ($v[0] === '<' && substr($v, -1) === '>') {
        if (preg_match('#^<(/)?(?:code|pre)(?:s[^<>]+?)?>$#', $v, $m)) {
            $x = isset($m[1]) && $m[1] === '/' ? 0 : 1;
        }
        $text .= $v; // this is a HTML tag…
    } else {
        $text .= !$x ? replace($v) : $v; // process or skip…
    }
}

return $text;

    Taken from here.