Are you asking how to write the regex pattern, or how to use a regex pattern in Kotlin? If you want help writing the pattern, this is probably not the right place to ask
(yes, pedants, in constrained scenarios regex is a fine tool for this job, but the better, more maintainable option is still going to be a proper XML parser)
This really doesn't belong here on the Kotlin slack. I'd suggest you either ask this in #random or on Stack overflow so that you're more likely to get a correct response