Update java-standalone-telemetry-processors.md

Author: jeanbisutti

articles/azure-monitor/app/java-standalone-telemetry-processors.md

@@ -260,6 +260,12 @@ The `mask` action requires the following settings:
 * `replace`
 * `action`: `mask`
 
+`pattern` must contain a named group placed betwen `?<` and `>:`. Example: `(?<userGroupName>[a-zA-Z.:\/]+)\d+`? The gorup is `(?<userGroupName>[a-zA-Z.:\/]+)` and `userGroupName` is the name of the group.
+
+`pattern` musts contain the same named group placed between `${` and `}` followed by the masK. Example where the mask is **: `${userGroupName}**`.
+
+See  [Telemetry processor examples](./java-standalone-telemetry-processors-examples.md) for the full example.
+
 ### Include criteria and exclude criteria
 
 Attribute processors support optional `include` and `exclude` criteria.