Update parser.py

Author: JBKahn

arrow/parser.py

@@ -45,7 +45,7 @@ class DateTimeParser(object):
     _TZ_NAME_RE = re.compile(r"\w[\w+\-/]+")
     # NOTE: timestamps cannot be parsed from natural language strings (by removing the ^...$) because it will
     # break cases like "15 Jul 2000" and a format list (see issue #447)
-    _TIMESTAMP_RE = re.compile(r"^\d+\.?\d+$")
+    _TIMESTAMP_RE = re.compile(r"^-?\d+\.?\d+$")
     _TIME_RE = re.compile(r"^(\d{2})(?:\:?(\d{2}))?(?:\:?(\d{2}))?(?:([\.\,])(\d+))?$")
 
     _BASE_INPUT_RE_MAP = {