Hi Jefferson,
I think you may need to escape the escape characters i.e.
should be
${string_2}= Set Variable "[\\x00-\\x7F]*C:\\\\Retail\\\\Software\\\\POSusb\\\\[\\x00-\\x7F]*"
The reason is escaped once for the string and again for the regex, this is a common problem in many testing tools and is not unique to Robot Framework
Hope that helps,
Dave.