You could create a "validate" metaobject with a validation rule (containing the regular expression), and then try to save the input to that metaobject. If it succeeds, you've validated your input.
I wouldn't really know any other way in which you can use regular expressions.
See this thread for an alternative solution which doesn't involve creating objects: https://forum.mendix.com/questions/393/Checking-if-a-value-matches-with-a-regular-expression-in-microflow