[New-bugs-announce] [issue11283] incorrect pattern in the re module docs for conditional regex

wesley chun report at bugs.python.org
Tue Feb 22 09:48:20 CET 2011


New submission from wesley chun <wescpy at gmail.com>:

In the re docs, it states the following for the conditional regular expression syntax:

(?(id/name)yes-pattern|no-pattern)
Will try to match with yes-pattern if the group with given id or name exists, and with no-pattern if it doesn’t. no-pattern is optional and can be omitted. For example, (<)?(\w+@\w+(?:\.\w+)+)(?(1)>) is a poor email matching pattern, which will match with '<user at host.com>' as well as 'user at host.com', but not with '<user at host.com'.

this regex is incomplete as it allows for 'user at host.com>':

>>> bool(re.match(r'(<)?(\w+@\w+(?:\.\w+)+)(?(1)>)', '<user at host.com>'))
True
>>> bool(re.match(r'(<)?(\w+@\w+(?:\.\w+)+)(?(1)>)', 'user at host.com'))
True
>>> bool(re.match(r'(<)?(\w+@\w+(?:\.\w+)+)(?(1)>)', '<user at host.com'))
False
>>> bool(re.match(r'(<)?(\w+@\w+(?:\.\w+)+)(?(1)>)', 'user at host.com>'))
True

This error has existed since this feature was added in 2.4...
http://docs.python.org/release/2.4.4/lib/re-syntax.html

... through the 3.3. docs...
http://docs.python.org/dev/py3k/library/re.html#regular-expression-syntax

The fix is to add the end char '$' to the regex to get all 4 working:


>>> bool(re.match(r'(<)?(\w+@\w+(?:\.\w+)+)(?(1)>|$)', '<user at host.com>'))
True
>>> bool(re.match(r'(<)?(\w+@\w+(?:\.\w+)+)(?(1)>|$)', 'user at host.com'))
True
>>> bool(re.match(r'(<)?(\w+@\w+(?:\.\w+)+)(?(1)>|$)', '<user at host.com'))
False
>>> bool(re.match(r'(<)?(\w+@\w+(?:\.\w+)+)(?(1)>|$)', 'user at host.com>'))
False

If accepted, I propose this patch (also attached):

$ svn diff re.rst
Index: re.rst
===================================================================
--- re.rst      (revision 88499)
+++ re.rst      (working copy)
@@ -297,9 +297,9 @@
 ``(?(id/name)yes-pattern|no-pattern)``
    Will try to match with ``yes-pattern`` if the group with given *id* or *name*
    exists, and with ``no-pattern`` if it doesn't. ``no-pattern`` is optional and
-   can be omitted. For example,  ``(<)?(\w+@\w+(?:\.\w+)+)(?(1)>)`` is a poor email
+   can be omitted. For example,  ``(<)?(\w+@\w+(?:\.\w+)+)(?(1)>|$)`` is a poor email
    matching pattern, which will match with ``'<user at host.com>'`` as well as
-   ``'user at host.com'``, but not with ``'<user at host.com'``.
+   ``'user at host.com'``, but not with ``'<user at host.com'`` nor ``'user at host.com>'`` .

----------
assignee: docs at python
components: Documentation, Regular Expressions
files: re.rst
messages: 129041
nosy: docs at python, wesley.chun
priority: normal
severity: normal
status: open
title: incorrect pattern in the re module docs for conditional regex
versions: Python 2.5, Python 2.6, Python 2.7, Python 3.1, Python 3.2, Python 3.3
Added file: http://bugs.python.org/file20833/re.rst

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<http://bugs.python.org/issue11283>
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