In the modeler one can define regular expressions, however these can't be used in the isMatch expression on an exclusive split.
My suggestion would be to support regular expressions as second argument in an isMatch() function within an exclusive split. See the screenshot below, there is a regular expression in the Project Explorer for EmailAddresses but this can't be used in the rule.
It may not be ideal, but you could store your regular expression as a constant and then put the following into your exclusive split:
I agree it is a little strange that REGEX values, which are pretty much just strings, can't be referenced in the platform as it they were constants.
I hope this is implemented quickly. Makes it much easier to use.