Hi Samuel you're right about the problem with the first regex but the second one seems foolproof to me.
I checked it with the regex testing tool http://www.solmetra.com/scripts/regex/index.php with the following options
Enter regular expression here:
Enter your data here:
Theme Name: Twenty Thirteen
Theme URI: http://wordpress.org/themes/twentythirteen
Author: the WordPress team
Author URI: http://wordpress.org/
Description: The 2013 theme for WordPress takes us back to the blog, featuring a full range of post formats, each displayed beautifully in their own unique way. Design details abound, starting with a vibrant color scheme and matching header images, beautiful typography and icons, and a flexible layout that looks great on any device, big or small.
License: GNU General Public License v2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
Tags: black, brown, orange, tan, white, yellow, light, one-column, two-columns, right-sidebar, flexible-width, custom-header, custom-menu, editor-style, featured-images, microformats, post-formats, rtl-language-support, sticky-post, translation-ready
Text Domain: twentythirteen
This theme, like WordPress, is licensed under the GPL.
Use it to make something cool, have fun, and share what you've learned with others.
As you said I separated the "Tags:" from the header comments block and put it below and looking at the  index of the Resultset: I see only the tag values.
The reason I'm stressing on this is because it misleads new developers. If someone forgets to put "Tags:" in the header comments and has a class or id selector with the string "tags" followed by a pseudo class they would get a WARNING telling them about a "wrong tag" instead of missing "Tags:"
When I got this error I thought that either somewhere the syntax of my CSS is wrong or this plugin has a problem with the
:before pseudo class.
If all this seems too much to do please include more details in the WARNING message for non-allowed tags like
WARNING: Found wrong tag, remove %s from the Tags: field in your style.css header.