[FIXED JENKINS-16660] Allow variable names with dots in bracketed references.

Given these <variable,value> mappings: <A,a> and <A.B,a-b>, $A.B would
evaluate to $a.B, as it currently does, and ${A.B} to a-b instead of the
current ${A.B}.

Existing ${A.B}-like references will break (not evaluate to ${A.B}) if
there actually is an A.B variable defined, which I think is very
unlikely.

core/src/main/java/hudson/Util.java

@@ -112,9 +112,9 @@ public class Util {
     }
 
     /**
-     * Pattern for capturing variables. Either $xyz or ${xyz}, while ignoring "$$"
+     * Pattern for capturing variables. Either $xyz, ${xyz} or ${a.b} but not $a.b, while ignoring "$$"
       */
-    private static final Pattern VARIABLE = Pattern.compile("\\$([A-Za-z0-9_]+|\\{[A-Za-z0-9_]+\\}|\\$)");
+    private static final Pattern VARIABLE = Pattern.compile("\\$([A-Za-z0-9_]+|\\{[A-Za-z0-9_.]+\\}|\\$)");
 
     /**
      * Replaces the occurrence of '$key' by <tt>properties.get('key')</tt>.