Use regex to crack RFC 3339 w/ nanosecond precision.

Avoids manual parsing, preseres initial strptime-generated error for
malformed timestamp strings.

Addresses:
#1569 (comment)

Author: tseaver

gcloud/_helpers.py

@@ -38,7 +38,16 @@
 
 _NOW = datetime.datetime.utcnow  # To be replaced by tests.
 _RFC3339_MICROS = '%Y-%m-%dT%H:%M:%S.%fZ'
-_RFC3339_NO_FRACTION_NO_ZULU = '%Y-%m-%dT%H:%M:%S'
+_RFC3339_NO_FRACTION = '%Y-%m-%dT%H:%M:%S'
+# datetime.strptime cannot handle nanosecond precision:  parse w/ regex
+_RFC3339_NANOS = re.compile(r"""
+    (?P<no_fraction>
+        \d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}  # YYYY-MM-DDTHH:MM:SS
+    )
+    \.                                       # decimal point
+    (?P<nanos>\d{9})                         # nanoseconds
+    Z                                        # Zulu
+""", re.VERBOSE)
 
 
 class _LocalStack(Local):
@@ -319,15 +328,14 @@ def _rfc3339_to_datetime(dt_str):
         return datetime.datetime.strptime(
             dt_str, _RFC3339_MICROS).replace(tzinfo=UTC)
     except ValueError:  # maybe nanosecond resoultion
-        prefix, suffix = dt_str.split('.')
-        fraction, zulu = suffix[:-1], suffix[-1]
-        if zulu == 'Z' and len(fraction) == 9:  # nanoseconds
-            nanos = int(fraction)
-            micros = nanos // 1000
-            bare_seconds = datetime.datetime.strptime(
-                prefix, _RFC3339_NO_FRACTION_NO_ZULU)
-            return bare_seconds.replace(microsecond=micros, tzinfo=UTC)
-        raise
+        with_nanos = _RFC3339_NANOS.match(dt_str)
+        if with_nanos is None:
+            raise
+        bare_seconds = datetime.datetime.strptime(
+            with_nanos.group('no_fraction'), _RFC3339_NO_FRACTION)
+        nanos = int(with_nanos.group('nanos'))
+        micros = nanos // 1000
+        return bare_seconds.replace(microsecond=micros, tzinfo=UTC)
 
 
 def _datetime_to_rfc3339(value):