ReReplacer makes it possible and easy to make your own little custom plugin-style tags.
This means you can place a simple little text like {mytag} in your content that (usually dynamically) gets replaced with something more complex.

The below examples will use this curly bracket syntax. But you can implement it in whatever syntax you want.
So instead of {mytag} you might want to use something like [[mytag]] or __MYTAG__ or something completely different.
Because YOU write what to search for, YOU decide what the syntax of the tag should be.

Icon tag

This tag makes it easy to place font based icons.

Icon packages like Icomoon or Font Awesome give you a great way of placing icons on your site.
However, if you want to place these in your content, you generally have to switch to the html view and mess around in there.
Also, your editor might not even show the icon and maybe even strip it away when saving the article or item.
This tag solves that.

PS: Here is an overview of all the Icomoon icons available in Joomla!

{icon joomla} {icon eye} {icon image} {icon star}

Search:

Enable the Regular Expressions option.

{icon (.*?)}

Replace:

<span class="icon-\1"></span>

Joomla Language string tag

This tag enables you to place language strings in your content. These will be passed through the Joomla language strings.

This requires the use of PHP code in the replacement, which is only available in the Pro version of ReReplacer.

Note: This will only work for language strings in language files that are loaded on the page. If you have your own custom language file you want to load, you need to force-load it in the php replacement.

{translate JGLOBAL_SECRETKEY_HELP}

Search:

Enable the Regular Expressions option.

{translate (.*?)}

Replace:

Enable the Replace with PHP option

<?php echo JText::_('\1'); ?>

Youtube video tag

{youtube IEe7wbxez34}

Search:

Enable the Regular Expressions option.

{youtube (.*?)}

This search is very simple, but also very 'stupid'. It will not remove any surrounding <p> tags. And it doesn't check for the correct syntax in the youtube id.
This is a more complex but better Regular Expressions search:

(?:<p>)?\{youtube(?:\s|&nbsp;|&#160;)+([a-z0-9-_]+)\}(?:</p>)?

Replace:

<iframe width="560" height="315" src="https://www.youtube.com/embed/\1" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe>

Vimeo video tag

Following the example for the Youtube tag above, you can do the same for a Vimeo tag.

{vimeo 347119375}

Search:

Enable the Regular Expressions option.

{vimeo (.*?)}

Or better:

(?:<p>)?\{vimeo(?:\s|&nbsp;|&#160;)+([a-z0-9]+)\}(?:</p>)?

Replace:

<iframe width="560" height="315" src="https://player.vimeo.com/video/\1" frameborder="0" allowfullscreen></iframe>

Youtube modal popup tag

Instead of placing the video directly into your content (like done in the examples above), you might want to have it pop up in a modal. You can use ReReplacer to output Modals tags.
Note: The plugin ordering is important. When using ReReplacer to output code that needs to be handled by other Joomla plugins, make sure you order the ReReplacer system plugin before the others.

{youtube IEe7wbxez34}

Search:

Enable the Regular Expressions option.

{youtube (.*?)}

Or better:

(?:<p>)?\{youtube(?:\s|&nbsp;|&#160;)+([a-z0-9-_]+)\}(?:</p>)?

Replace:

{modal url="https://www.youtube.com/embed/\1?autoplay=1&rel=0"}
<img src="//img.youtube.com/vi/\1/mqdefault.jpg" alt="" />
{/modal}

Vimeo modal popup tag

Following the example for the Youtube tag above, you can do the same for a Vimeo tag.

{vimeo 347119375}

Search:

Enable the Regular Expressions option.

{vimeo (.*?)}

Or better:

(?:<p>)?\{vimeo(?:\s|&nbsp;|&#160;)+([a-z0-9]+)\}(?:</p>)?

Replace:

Note: There is no way to directly link to the Vimeo thumbnail image. So you can either create your own, or use this custom php script.

{modal url="https://player.vimeo.com/video/\1?autoplay=1"}
<img src="/path/to/my/vimeo_image.php?id=\1" alt="" />
{/modal}

User Table tag

Another example using PHP code in the replacement, which is only available in the Pro version of ReReplacer.

By enabling the "Replace with PHP" setting, you can replace simple words or pieces of html with complete php code snippets.

{usertable Peter van Westen}

Search:

Enable the Regular Expressions option.

\{usertable (.*?)\}

Replace:

Enable the Replace with PHP option.

<?php
$query = $db->getQuery(true)
   ->select('name, email, registerDate')
   ->from('#__users')
   ->where('id = ' . $db->q('\1') . ' OR username = ' . $db->q('\1') . ' OR name = ' . $db->q('\1'));
$db->setQuery($query);
$user = $db->loadObject();
if ($user) {
    echo '
        <table>
            <tr>
                <td>Name:</td>
                <td>' . $user->name . '</td>
            </tr>
            <tr>
                <td>Email:</td>
                <td>' . $user->email . '</td>
            </tr>
            <tr>
                <td>Registered on:</td>
                <td>' . $user->registerDate . '</td>
            </tr>
        </table>
    ';
}
?>

For long blocks of php/html you can also choose to place the code in an external file and simply include it.

So in the replace field you can place:

<?php
$myvar = '\1';
include JPATH_SITE . '/myfiles/file.php';
?>

With the above code, you can use the variable $myvar in the external file. The variable will contain whatever you placed inside the {usertable} tag in your article. Of course, you can name the variable however you want.