[deathanatos]

A regex only seems to take ~1µs.

  In [7]: iso_regex = re.compile('(\\d{4})-(\\d{2})-(\\d{2})T(\\d{2}):(\\d{2}):(\\d{2}(?:\\.?\\d+))')

  In [8]: %timeit iso_regex.match('2014-01-09T21:48:00.921000')
  1000000 loops, best of 3: 1.05 µs per loop

But hey, once it's written in C, why go back?

I'm missing the timezone, but the OP left that out, so I did too. For comparison, dateutil's parse takes ~76µs for me. Kinda makes me wonder why aniso8601 is so slow. (It's also missing a few other things, depending on if you count all the non-time forms as valid input.)

That said, cool! I might use this. One of the things that makes dateutil's parse slower is that it'll parse more than just ISO-8601: it parses many things that look like dates, including some very non-intuitive ones that have caused "bugs"¹. Usually in APIs, its "dates are always ISO-8601", and all I really need is an ISO-8601 parser. While I appreciate the theory behind "be liberal in what you accept", sometimes, I'd rather error out than to build expectations that sending garbage — er, stuff that requires a complicated parse algorithm that I don't really understand — is okay.

¹dateutil.parser.parse('') is midnight of the current date. Why, I don't know. Also, dateutil.parser.parse('noon') is "TypeError: 'NoneType' object is not iterable".

[ajanuary]

The library has the following features your regex is missing:

* Every part from month onwards is optional

* Separator characters are optional

* Date/time separator can be a space as well as T

* Timezone information

* Parsing the strings into numbers

* Actually creates a datetime object

I expect adding all of those will bump up the time a bit.

[ajanuary]

I'm not much of a regex wizard, but I tried to add all the features listed other than parsing the result and creating the datetime object.

    iso_regex = re.compile('([0-9]{4})-?([0-9]{1,2})(?:-?([0-9]{1,2})(?:[T ]([0-9]{1,2})(?::?([0-9]{1,2})(?::?([0-9]{1,2}(?:\\.?[0-9]+)?))?(?:(Z)|([+-][0-9]{1,2}):?([0-9]{1,2})))?)?)?')

It seems like it performs quite a bit worse than the library, which creates the full object.

    In [82]: %timeit ciso8601.parse_datetime('2014-01-09T21:48:00.921000')
    1000000 loops, best of 3: 368 ns per loop

    In [83]: %timeit iso_regex.match('2014-01-09T21:48:00.921000')
    100000 loops, best of 3: 9.72 µs per loop

In the interest of intellectual pursuit, is there anything that can be done to the regex to speed it up?

[randlet]

Note you still need to convert your regex match to a datetime object which is likely going to add some significant overhead.

[thomas-st]

Good idea with the regex, haven't tried it. That being said, you didn't take the time into account to construct a datetime object, let alone attach time zone information. ciso8601 supports time zones using pytz' FixedOffset and UTC classes, see https://github.com/elasticsales/ciso8601 for additional benchmarks. There's potential for further speedup by using a tzoffset subclass written in C, but in our cases all dates were UTC anyway and so we didn't need the time zone.