Unfortunately openHAB REGEX doesn’t behave the way standard REGEX does. For one your REGEX must match the entire message, hence Vincent’s recommendation of adding the .().. Second, you can only have one match (i.e. one ()). If you have multiple matching groups there is no way to tell the REGEX transform which one you actually want. That transform appears to have two matching groups. Finally, you must supply a matching group that defines what part you want from the string.
It looks like what you want is the stuff between "summary":" and ","updated" so the REGEX should look something like:
REGEX(.*"summary":"(.*)",\"updated.*)
The .* matches everything up to the "summary":", the "summary":" is the beginning marker for the text we actually care about, the (.*) matches and returns the text you want to extract, the "updated is the ending marker for the text we actually care about and the .* matches the rest of the String.