PEP 440 Python Versioning Compliance/Parsing

version.py

#! /usr/bin/env python
# -*- coding: utf-8 -*-

import sys
import re

VERSION_PATTERN = r"""
    v?
    (?:
        (?:(?P<epoch>[0-9]+)!)?                           # epoch
        (?P<release>[0-9]+(?:\.[0-9]+)*)                  # release segment
        (?P<pre>                                          # pre-release
            [-_\.]?
            (?P<pre_l>(a|b|c|rc|alpha|beta|pre|preview))
            [-_\.]?
            (?P<pre_n>[0-9]+)?
        )?
        (?P<post>                                         # post release
            (?:-(?P<post_n1>[0-9]+))
            |
            (?:
                [-_\.]?
                (?P<post_l>post|rev|r)
                [-_\.]?
                (?P<post_n2>[0-9]+)?
            )
        )?
        (?P<dev>                                          # dev release
            [-_\.]?
            (?P<dev_l>dev)
            [-_\.]?
            (?P<dev_n>[0-9]+)?
        )?
    )
    (?:\+(?P<local>[a-z0-9]+(?:[-_\.][a-z0-9]+)*))?       # local version
"""

_regex = re.compile(
    r"^\s*" + VERSION_PATTERN + r"\s*$",
    re.VERBOSE | re.IGNORECASE,
)

def is_canonical(version):
    return re.match(r'^([1-9]\d*!)?(0|[1-9]\d*)(\.(0|[1-9]\d*))*((a|b|rc)(0|[1-9]\d*))?(\.post(0|[1-9]\d*))?(\.dev(0|[1-9]\d*))?$', version) is not None

def extract(version):
    ok = is_canonical(version)
    if not ok:
        sys.exit("Not compliant")
    m = _regex.match(version)
    if m:
        for key in ("epoch", "release", "pre", "pre_l", "pre_n", "post", "post_n1", "post_l", "post_n2", "dev", "dev_l", "dev_n", "local"):
            print("{:>15}: {}".format(key, m.group(key)))
    else:
        print("No match")

version = sys.argv[1]
extract(version)