Found the response.
In fact, you don’t have to key the REGEX() word for the 5th parameter. You just have to key the REGEX rules directly.
String LuminositeSensor "Luminosité [%s]" (g1) {mqtt="<[mqttserver:domoticz/out:state:JSONPATH($.svalue1):.*\"Luminosite\".*]"}
The sample in the wiki make me the mistake.