WordPress plugin: finding the <!--more--> in the_content - filter

Question

I'm writing a WordPress plugin that filters the_content, and I'd like to make use of the <!--more--> tag, but it appears that it has been stripped out by the time it reaches me. This appears to be not a filter, but a function of the way WordPress works.
I could of course resort to reloading the already-loaded content from the database, but that sounds like it might cause other troubles. Is there any good way for me to get the raw content without the <!--more--> removed?

Answer 1

Chances are, by the time your plugin runs, <!--more--> has been converted to <span id="more-1"></span>
This is what I use in my plugin, which injects some markup immediately after the <!--more--> tag:
add_filter('the_content', 'inject_content_filter', 999);
function inject_content_filter($content) {
$myMarkup = "my markup here<br>";
$content = preg_replace('/<span id\=\"(more\-\d+)"><\/span>/', '<span id="\1"></span>'."\n\n". $myMarkup ."\n\n", $content);
return $content;
}

Answer 2

You can use the follow code:
The !is_single() will avoid display the more link in the View Post page.
add_filter('the_content', 'filter_post_content');
function filter_post_content($content,$post_id='') {
if ($post_id=='') {
global $post;
$post_id = $post->ID;
}
// Check for the "more" tags
$more_pos = strpos($filtered_content, '<!--more-->');
if ($more_pos && !is_single()) {
$filtered_content = substr($filtered_content, 0, $more_pos);
$replace_by = '<a href="' . get_permalink($post_id) . '#more-' . $post_id
. '" class="more-link">Read More <span class="meta-nav">→</span></a>';
$filtered_content = $filtered_content . $replace_by;
}
return $filtered_content;
}

Answer 3

Based on Frank Farmer's answer I solved to add thumbnail photo after the generated more tag (<span id="more-...) in single.php file with this:
// change more tag to post's thumbnail in single.php
add_filter('the_content', function($content)
{
if(has_post_thumbnail())
{
$post_thumbnail = get_the_post_thumbnail(get_the_ID(), 'thumbnail', array('class'=>'img img-responsive img-thumbnail', 'style'=>'margin-top:5px;'));
$content = preg_replace('/<span id\=\"(more\-\d+)"><\/span>/', '<span id="\1"></span>'.$post_thumbnail, $content);
}
return $content;
}, 999);