Draft: advanced indexing in Skip for Array

syntax aligned with Part_CompoundFilter syntax. With the extension that
index patterns can be intermixed with manual enumeration in a matlab style:
	[1, 3:6:2] -> [1. 3, 5]

src/Mod/Draft/draftobjects/array.py

@@ -35,6 +35,7 @@
 # @{
 import math
 import json
+import re
 from PySide.QtCore import QT_TRANSLATE_NOOP
 
 import FreeCAD as App
@@ -389,29 +390,6 @@ def show_and_hide(self, obj, prop):
                            "NumberPolar", "Angle", "IntervalAxis"):
                     obj.setPropertyStatus(pr, "Hidden")
 
-    def parse_skip_list(self, obj):
-        """
-            Parses obj.Skip as a list of integers.
-            The reason for parsing this ourselves instead of using PropertyIntegerList
-            is so that we can handle failure silently
-        """
-        try:
-            skip_list_raw = json.loads(obj.Skip)
-        except json.decoder.JSONDecodeError:
-            return []
-
-        if not isinstance(skip_list_raw, list):
-            return []
-
-        skip_list = []
-
-        for s in skip_list_raw:
-            if isinstance(s, int):
-                skip_list.append(s)
-
-        return skip_list
-
-
     def execute(self, obj):
         """Execture when the object is created or recomputed."""
         if not obj.Base:
@@ -432,7 +410,7 @@ def execute(self, obj):
                          "reference.")
                 raise TypeError(_info)
 
-        skip_list = self.parse_skip_list(obj)
+        skip_list = parse_skip_list(obj.Skip)
 
         if obj.ArrayType == "ortho":
             pls = rect_placements(obj.Base.Placement,
@@ -577,4 +555,53 @@ def circ_placements(base_placement,
 
     return placements
 
+
+def parse_skip_list(skip_expression):
+    """
+        Parses a skip expression (as found in obj.Skip) as a list of integers.
+
+        The reason for parsing this ourselves instead of using PropertyIntegerList
+        is so that we can handle failure silently, and support more advanced indexing.
+
+        Syntax:
+        ========
+
+        Skip expressions are expanded in a matlab like way
+
+        Example 1: skip_expression = 1:5
+            Will be expanded to [1, 2, 3, 4, 5]
+
+        Example 2: skip_expression = [2:5, 9]
+            Will be expanded to [2, 3, 4, 5, 9]
+
+        Example 3: skip_expression = [1, 3:2:7, 9]
+            Will be expanded to [1, 3, 5, 7, 9]
+    """
+
+    matches = re.finditer(r"(?P<A>\d+):(?P<B>\d+)(:(?P<C>\d+))*", skip_expression)
+
+    skip_list = []
+    for m in matches:
+        d = m.groupdict()
+        d = {k:None if d[k] is None else int(d[k]) for k in d}
+        if d['C']:
+            skip_list.extend(range(d['A'], d['B'], d['C']))
+        else:
+            skip_list.extend(range(d['A'], d['B']))
+        skip_expression = skip_expression.replace(m.group(), '-0.1')  # hack: we use a float as a marker in order have valid json, but not a valid index.
+
+    try:
+        skip_list_raw = json.loads(skip_expression)
+    except json.decoder.JSONDecodeError:
+        return skip_list
+
+    if not isinstance(skip_list_raw, list):
+        return skip_list
+
+    for s in skip_list_raw:
+        if isinstance(s, int):
+            skip_list.append(s)
+
+    return skip_list
+
 ## @}