PHP Regex expression excluding <pre> tag - php

Question

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

Answer 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$acronym\b
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>ASCII\nSometext\nMoretext</pre>More text \nASCII\nMore text<pre>More\nlines\nASCII\nlines</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>

Answer 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('"', '"', $s); // do something with `$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.