config: Fix parsing posargs default with colon

e4d0d60 introduced a regression when posargs default
contained a colon, as the common substitution arg
regex splits on the colon, and uses different replace
types depending on the number of colons found.

Fixes #1785

docs/changelog/1785.bugfix.rst

@@ -0,0 +1 @@
+Fix regression parsing posargs default containing colon. - by :user:`jayvdb`

src/tox/config/__init__.py

@@ -1852,6 +1852,13 @@ def _replace_match(self, match):
             return self.reader.getposargs(default_value)
 
         sub_type = g["sub_type"]
+        if sub_type == "posargs":
+            if default_value:
+                value = "{}:{}".format(sub_value, default_value)
+            else:
+                value = sub_value
+            return self.reader.getposargs(value)
+
         if not sub_type and not sub_value:
             raise tox.exception.ConfigError(
                 "Malformed substitution; no substitution type provided. "

tests/unit/config/test_config.py

@@ -685,7 +685,7 @@ def test_command_substitution_from_other_section_posargs(self, newconfig):
             """,
         )
         reader = SectionReader("testenv", config._cfg)
-        reader.addsubstitutions([r"argpos"])
+        reader.addsubstitutions(["argpos"])
         x = reader.getargvlist("commands")
         assert x == [["thing", "argpos", "arg2"]]
 
@@ -701,10 +701,23 @@ def test_command_section_and_posargs_substitution(self, newconfig):
             """,
         )
         reader = SectionReader("testenv", config._cfg)
-        reader.addsubstitutions([r"argpos"])
+        reader.addsubstitutions(["argpos"])
         x = reader.getargvlist("commands")
         assert x == [["thing", "arg1", "argpos", "endarg"]]
 
+    def test_command_posargs_with_colon(self, newconfig):
+        """Ensure posargs with default containing : succeeds"""
+        config = newconfig(
+            r"""
+            [testenv]
+            commands =
+                pytest {posargs:default with : colon after}
+            """,
+        )
+        reader = SectionReader("testenv", config._cfg)
+        x = reader.getargvlist("commands")
+        assert x[0] == ["pytest", "default", "with", ":", "colon", "after"]
+
     def test_command_missing_substitution(self, newconfig):
         config = newconfig(
             """
@@ -747,8 +760,38 @@ def test_command_env_substitution_posargs(self, newconfig):
         """,
         )
         envconfig = config.envconfigs["py27"]
-        assert envconfig.commands == [["ls", "default"]]
         assert envconfig.setenv["TEST"] == "default"
+        assert envconfig.commands == [["ls", "default"]]
+
+    def test_command_env_substitution_posargs_with_colon(self, newconfig):
+        """Ensure {posargs} values are substituted correctly."""
+        config = newconfig(
+            """
+           [testenv:py27]
+           setenv =
+             TEST=pytest {posargs:default with:colon after}
+           commands =
+             ls {env:TEST}
+        """,
+        )
+        envconfig = config.envconfigs["py27"]
+        assert envconfig.setenv["TEST"] == "pytest default with:colon after"
+        assert envconfig.commands == [["ls", "pytest", "default", "with:colon", "after"]]
+
+    def test_command_env_substitution_posargs_with_spaced_colon(self, newconfig):
+        """Ensure {posargs} values are substituted correctly."""
+        config = newconfig(
+            """
+           [testenv:py27]
+           setenv =
+             TEST=pytest {posargs:default with : colon after}
+           commands =
+             ls {env:TEST}
+        """,
+        )
+        envconfig = config.envconfigs["py27"]
+        assert envconfig.setenv["TEST"] == "pytest default with : colon after"
+        assert envconfig.commands == [["ls", "pytest", "default", "with", ":", "colon", "after"]]
 
     def test_command_env_substitution_global(self, newconfig):
         """Ensure referenced {env:key:default} values are substituted correctly."""