Steven Vachon

If you were to use this on a website like http;//example.com/test/testing/, you would get results like these:
<ol>
<li>
<ul>
<li>Before: http;//example.com/test/another-test/#anchor</li>
<li>After: ../another-test/#anchor</li>
</ul>
</li>
<li>
<ul>
<li>Before: http;//example.com/wp-content/themes/twentyten/style.css</li>
<li>After: /wp-content/themes/twentyten/style.css</li>
</ul>
</li>
<li>
<ul>
<li>Before: https;//example.com/wp-content/themes/twentyten/style.css</li>
<li>After: https;//example.com/wp-content/themes/twentyten/style.css</li>
</ul>
</li>
<li>
<ul>
<li>Before: http;//google.com/test/</li>
<li>After: //google.com/test/</li>
</ul>
</li>
<li>
<ul>
<li>Before: ../../../../../../../../#anchor</li>
<li>After: /#anchor</li>
<li>After (<code>$output_type=1</code>): ../../#anchor</li>
</ul>
</li>
</ol>
All string parsing. No directory browsing.
If you’re looking for a plugin that will automatically convert all URLs on your WordPress site, instead check out my other plugin, <a href="https://wordpress.org/extend/plugins/wp-html-compression/" rel="ugc">WP-HTML-Compression</a>. 
Before you copy this code and add it into your own, keep in mind that there will probably be future updates. Keeping the code within an installed plugin will make sure that you’re notified.

Absolute-to-Relative URLs

A function() for use in shortening URL links. This plugin is meant for dev work and does not automatically shorten URLs.

tufto

Before: 
http://example.com/wp-content/themes/twentyten/style.css
After: 
/wp-content/themes/twentyten/style.css
OR:
Before: 
http://example.com/wp-content/themes/twentyten/style.css
After (adding “https”): 
https://example.com/wp-content/themes/twentyten/style.css
If you are looking for a plugin that automatically converts absolute URLs to relative URLs, or for a plugin that changes URLS for src & href URL attributes check out my other plugin.
I wrote this plugin because I needed a basepath for uploaded images for my WordPress Multisite. In other words, I needed a URL to the server root without the domain. You may find this useful for the same reason, or perhaps another.
Before you copy this code and add it into your own, keep in mind that there may be future updates. Keeping the code within an activated plugin will make sure you’re notified.

i=== URLs Editor

A FUNCTION for use in shortening URL links. This plugin automatically does shorten URLs.

Absolute-to-Relative URLs vs i=== URLs Editor