Provide examples regular expression for pep 440. (#227)

* Provide examples regular expression for pep 440.

Closes #226

pep-0440.txt

@@ -94,6 +94,11 @@ this scheme but MUST also include the normalizations specified below.
 Installation tools MAY warn the user when non-compliant or ambiguous versions
 are detected.
 
+See also `Appendix B : Parsing version strings with regular expressions` which
+provides a regular expression to check strict conformance with the canonical
+format, as well as a more permissive regular expression accepting inputs that
+may require subsequent normalization.
+
 Public version identifiers are separated into up to five segments:
 
 * Epoch segment: ``N!``
@@ -1575,6 +1580,62 @@ Metadata v2.0 guidelines versus setuptools::
     Projects with No Compatible Versions:     498/47114 (1.06%)
     Projects with Differing Latest Version:   688/47114 (1.46%)
 
+Appendix B : Parsing version strings with regular expressions
+=============================================================
+
+As noted earlier in the `Public version identifiers` section, published
+version identifiers SHOULD use the canonical format. This section provides
+regular expressions that can be used to test whether a version is already
+in that form, and if it's not, extract the various components for subsequent
+normalization.
+
+To test whether a version identifier is in the canonical format, you can use
+the following function::
+
+    import re
+    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*))?$', version) is not None
+
+To extract the components of a version identifier, use the following regular
+expression (as defined by the `packaging <https://github.com/pypa/packaging>`_
+project)::
+
+    VERSION_PATTERN = r"""
+        v?
+        (?:
+            (?:(?P<epoch>[0-9]+)!)?                           # epoch
+            (?P<release>[0-9]+(?:\.[0-9]+)*)                  # release segment
+            (?P<pre>                                          # pre-release
+                [-_\.]?
+                (?P<pre_l>(a|b|c|rc|alpha|beta|pre|preview))
+                [-_\.]?
+                (?P<pre_n>[0-9]+)?
+            )?
+            (?P<post>                                         # post release
+                (?:-(?P<post_n1>[0-9]+))
+                |
+                (?:
+                    [-_\.]?
+                    (?P<post_l>post|rev|r)
+                    [-_\.]?
+                    (?P<post_n2>[0-9]+)?
+                )
+            )?
+            (?P<dev>                                          # dev release
+                [-_\.]?
+                (?P<dev_l>dev)
+                [-_\.]?
+                (?P<dev_n>[0-9]+)?
+            )?
+        )
+        (?:\+(?P<local>[a-z0-9]+(?:[-_\.][a-z0-9]+)*))?       # local version
+    """
+
+    _regex = re.compile(
+        r"^\s*" + VERSION_PATTERN + r"\s*$",
+        re.VERBOSE | re.IGNORECASE,
+    )
+
 
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