How can I simply extract a variable with Rascal Regex

Question

If I process a simple string with a regex, I expect I can extract variables. The examples in the manual states that the extraction results in a stored variable. This does not work as expected. If I do the following regex:

/\w*<subtext:text>\w*/ := "myfulltextstring"

I would expect the variable subtext to contain the string text. But it is undeclared. If I declare subtext before executing, it is empty. What is the simple way to do this extraction?