improved lexer's regular expressions

- made them mutually exclusive, such that we don't face the keyword args ordering issue anymore
- fixed some unhandled corner cases (added appropriate test cases as well)
  - handle $$( additionally to $${
  - handle text in front of $$ or single $

.travis.yml

@@ -3,6 +3,7 @@ sudo: enabled
 language: python
 python:
   - "2.7"
+  - "3.5"
   - "3.6"
 
 # command to install dependencies

src/xacro/__init__.py

@@ -573,13 +573,10 @@ def grab_properties(elt, table):
         elt = next
 
 
-# TODO: The order of arguments here is relevant!
-# However, python 3.0 - 3.5 doesn't preserve order of keyword arguments
-# This is only fixed with https://www.python.org/dev/peps/pep-0468 in 3.6
-LEXER = QuickLexer(DOLLAR_DOLLAR_BRACE=r"\$\$+\{",
-                   EXPR=r"\$\{[^\}]*\}",
-                   EXTENSION=r"\$\([^\)]*\)",
-                   TEXT=r"([^\$]|\$[^{(]|\$$)+")
+LEXER = QuickLexer(DOLLAR_DOLLAR_BRACE=r"^\$\$+(\{|\()", # multiple $ in a row, followed by { or (
+                   EXPR=r"^\$\{[^\}]*\}",       # stuff starting with ${
+                   EXTENSION=r"^\$\([^\)]*\)",  # stuff starting with $(
+                   TEXT=r"[^$]+|\$[^{($]+|\$$") # any text w/o $  or  $ following any chars except {($  or  single $
 
 
 # evaluate text and return typed value

test/test_xacro.py

@@ -447,13 +447,13 @@ def test_substitution_args_arg(self):
 
     def test_escaping_dollar_braces(self):
         self.assert_matches(
-                self.quick_xacro('''<a b="$${foo}" c="$$${foo}" />'''),
-                '''<a b="${foo}" c="$${foo}" />''')
+                self.quick_xacro('''<a b="$${foo}" c="$$${foo}" d="text $${foo}" e="text $$${foo}" f="$$(pwd)" />'''),
+                '''<a b="${foo}" c="$${foo}" d="text ${foo}" e="text $${foo}" f="$(pwd)" />''')
 
     def test_just_a_dollar_sign(self):
         self.assert_matches(
-                self.quick_xacro('''<a b="$" />'''),
-                '''<a b="$" />''')
+                self.quick_xacro('''<a b="$" c="text $" d="text $ text"/>'''),
+                '''<a b="$" c="text $" d="text $ text"/>''')
 
     def test_multiple_insert_blocks(self):
         self.assert_matches(