Merge branch 'master' of git://github.com/flosch/mongoengine into v0.4

Conflicts:
	tests/fields.py

mongoengine/fields.py

@@ -66,6 +66,9 @@ class StringField(BaseField):
                 regex = r'%s$'
             elif op == 'exact':
                 regex = r'^%s$'
+
+            # escape unsafe characters which could lead to a re.error
+            value = re.escape(value)
             value = re.compile(regex % value, flags)
         return value