Fix dev constraint parsing

Fixes #206

poetry/semver/__init__.py

@@ -119,6 +119,9 @@ def parse_single_constraint(constraint):  # type: (str) -> VersionConstraint
         op = m.group(1)
         version = m.group(2)
 
+        if version == "dev":
+            version = "0.0-dev"
+
         try:
             version = Version.parse(version)
         except ValueError:

poetry/semver/patterns.py

@@ -15,5 +15,5 @@ TILDE_CONSTRAINT = re.compile("(?i)^~(?!=)({})$".format(_COMPLETE_VERSION))
 TILDE_PEP440_CONSTRAINT = re.compile("(?i)^~=({})$".format(_COMPLETE_VERSION))
 X_CONSTRAINT = re.compile("^(!=|==)?\s*v?(\d+)(?:\.(\d+))?(?:\.(\d+))?(?:\.[xX*])+$")
 BASIC_CONSTRAINT = re.compile(
-    "(?i)^(<>|!=|>=?|<=?|==?)?\s*({})".format(_COMPLETE_VERSION)
+    "(?i)^(<>|!=|>=?|<=?|==?)?\s*({}|dev)".format(_COMPLETE_VERSION)
 )

tests/semver/test_main.py

@@ -24,6 +24,7 @@ from poetry.semver import VersionUnion
         ("=1.0", Version(1, 0, 0)),
         ("1.2.3b5", Version(1, 2, 3, "b5")),
         (">= 1.2.3", VersionRange(min=Version(1, 2, 3), include_min=True)),
+        (">dev", VersionRange(min=Version(0, 0, pre="dev"))),  # Issue 206
     ],
 )
 def test_parse_constraint(input, constraint):