Python Regex doesn't work as expected

Question

i've crafted this regular expression:

<entry>\\n<(\w+)>(.+?)</\w+>\\n</entry>

to parse the following RSS Feed:

<?xml version="1.0" encoding="UTF-8"?>\n<feed version="0.3" xmlns="http://purl.org/atom/ns#">\n<title>Gmail - Inbox for [email protected]</title>\n<tagline>New messages in your Gmail Inbox</tagline>\n<fullcount>2</fullcount>\n<link rel="alternate" href="http://mail.google.com/mail" type="text/html" />\n<modified>2011-03-15T11:07:48Z</modified>\n<entry>\n<title>con due mail...</title>\n<summary>Gianluca Bargelli http://about.me/proudlygeek/bio</summary>\n<link rel="alternate" href="http://mail.google.com/[email protected]&amp;message_id=12eb9332c2c1fa27&amp;view=conv&amp;extsrc=atom" type="text/html" />\n<modified>2011-03-15T11:07:42Z</modified>\n<issued>2011-03-15T11:07:42Z</issued>\n<id>tag:gmail.google.com,2004:1363345158434847271</id>\n<author>\n<name>me</name>\n<email>[email protected]</email>\n</author>\n</entry>\n<entry>\n<title>test nuova mail</title>\n<summary>Gianluca Bargelli sono tornato!?& http://about.me/proudlygeek/bio</summary>\n<link rel="alternate" href="http://mail.google.com/[email protected]&amp;message_id=12eb93140d9f7627&amp;view=conv&amp;extsrc=atom" type="text/html" />\n<modified>2011-03-15T11:05:36Z</modified>\n<issued>2011-03-15T11:05:36Z</issued>\n<id>tag:gmail.google.com,2004:1363345026546890279</id>\n<author>\n<name>me</name>\n<email>[email protected]</email>\n</author>\n</entry>\n</feed>\n'skinner.com/products/spl].

The problem is that i am not getting any matches by using Python's re module:

import re

regex = re.compile("""<entry>\\n<(\w+)>(.+?)</\w+>\\n</entry>""")
regex.findall(rss_string) # Returns an empty list

Using an online regex tester (such as this) works as expected, so i don't think is a regex problem.

Edit

I am well aware that using regular expressions to parse a Context-Free Grammar is BAD, but in my case the regular expression is likely to work only for that RSS feed (it is a Gmail inbox feed, by the way) and i know i can use an external library/xml parser for this task: it is only an exercise, not an habit.

The question should be Why the following regular expression don't work as expected in Python?

Answer 1

Before the regex compiler sees a string, Python has already processed the slash-escapes, therefore you'd have to escape it twice (e.g. \\\\n for \\n). However, Python has a handy notation for exactly this sort of thing, just stick an r before the string:

regex = re.compile(r"""<entry>\\n<(\w+)>(.+?)</\w+>\\n</entry>""")

By the way, I agree with the others here, do not use regexes to parse XML. However, hopefully you will find this string notation helpful in future regular expressions.