Why won't my preg_replace work with content_save_pre?

I copied your code to a file in my home directory on my Ubuntu box, removed the weird character that @s_ha_dum noticed, and ran it. It worked as I’d expect.

Code:

<?php

function get_the_post_imgs($oldPostContent) {
    $newPostContent = $oldPostContent;
    $newPostContent = preg_replace_callback(
        "/<img.*src=[\"']([^\"']*)[\"'].*>/i", // pattern to match to (i.e the contents of an <img src="https://wordpress.stackexchange.com/questions/140017/..." /> tag)
        function ($match) {
            return "hello world"; // obviously it's a lot more complicated than this but for your sake i've massively simplified it so the src="https://wordpress.stackexchange.com/questions/140017/..." content is just replaced with "hello world" for now
        },
        $oldPostContent
    );

    return $newPostContent; // return new manipulated content
}

echo get_the_post_imgs( '<a href="https://wordpress.stackexchange.com/questions/140017/foo.php"><img align="right" src="http://example.com/fire.gif" /></a><br />' );

?>

Running this code from the command line:

% php -e foo.php 
<a href="https://wordpress.stackexchange.com/questions/140017/foo.php">hello world

Your regex might be a little greedy for your purposes, but it works.

PS, trying to run your code with the weird character still in place:

% php -e foo.php
PHP Parse error:  syntax error, unexpected '"/<img.*src=[\"']([^\"']*)[\"'' (T_CONSTANT_ENCAPSED_STRING) in /home/username/foo.php on line 6

Edit — Since you asked in the comments, I thought I’d try to cobble up a less greedy regex for you. Try this on for size:

<img.*src=[\"']([^\"']*)[\"'][^>]*>

The extra [^>*] term at the end ends the <img...> tag at its closing >, instead of carrying forward to the last > in the line (in my example, the closing > in the </a> tag).

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