There is no way to do this in the microflow itself but you can call a simple java action that you pass the regular expression and the string to be verified. You could define this regular expression in the project constants.
This is the code:
Pattern pattern = Pattern.compile(regexString);
Matcher matcher = pattern.matcher(stringToValidate);
boolean isValid = matcher.matches();
Or the short version for a java action returning the boolean:
return Pattern.matches(regexString, stringToValidate);