The regular expression inside there is: ^cl_(.*).lua and that will work fine in Opus.
The / ... /lg part surrounding the regular expression is not part of regex syntax. You are using a programming language's syntax for applying regular expression operations and options, which is language-specific and not part of actual regular expressions. It's similar to adding some Perl or JavaScript code and function calls into the regex field and expecting it to work. The regex itself will work, but you need to remove the extra code around it which isn't actually part of the regular expression syntax.
You could use the Find panel to find all files below those folders and/or with the labels you want to clear, then select-all and clear them.
Recursing into selected folders to remove/reset a label from all files, after applying it to individual files (not a wildcard etc.), could be done using a script but isn't built in.
Which config file are we talking about here?
I guess it also depends what you mean by human readable. I can read this one:
<?xml version="1.0" encoding="UTF-8"?>
<filecolors>
<color bg_sel="#316ac5" bg_unsel="none" class="bakfile" fg_sel="#800080" fg_unsel="#800080" type=".bak" />
</filecolors>
But also note that individually applied labels may be stored in NTFS metadata rather than config files, depending on how you have Opus configured. Storing them in metadata means the labels stay with the files when they are moved or renamed (whether in Opus or in something else), while storing them in the config means the labels always point to a particular path/name (or wildcard etc.) which doesn't move with the file. Both modes are useful for different things, and can be mixed and matched.
If we are talking about labels stored in NTFS metadata then there isn't really a config file at all for you to edit; the data is in the filesystem.