Beginning Python - From Novice to Professional

244 CHAPTER 10 ■ BATTERIES INCLUDED
GREEDY AND NONGREEDY PATTERNS
The repetition operators are by default greedy; that means that they will match as much as possible. For
example, let’s say I rewrote the emphasis program to use the following pattern:
>>> emphasis_pattern = r'\*(.+)\*'
This matches an asterisk, followed by one or more letters, and then another asterisk. Sounds perfect,
doesn’t it? But it isn’t:
>>> re.sub(emphasis_pattern, r'\1', '*This* is *it*!')
'This* is *it!'
As you can see, the pattern matched everything from the first asterisk to the last—including the two
asterisks between! This is what it means to be greedy: Take everything you can.
In this case, you clearly don’t want this overly greedy behavior. The solution presented in the preceding
text (using a character set matching anything except an asterisk) is fine when you know that one specific letter
is illegal. But let’s consider another scenario: What if you used the form '**something**' to signify emphasis?
Now it shouldn’t be a problem to include single asterisks inside the emphasized phrase. But how do you avoid
being too greedy?
Actually, it’s quite easy; you just use a nongreedy version of the repetition operator. All the repetition
operators can be made nongreedy by putting a question mark after them:
>>> emphasis_pattern = r'\*\*(.+?)\*\*'
>>> re.sub(emphasis_pattern, r'\1', '**This** is **it**!')
'This is it!'
Here I’ve used the operator +? instead of +, which means that the pattern will match one or more occurrences
of the wildcard, as before. However, it will match as few as it can, because it is now nongreedy; it will
match only the minimum needed to reach the next occurrence of '\*\*', which is the end of the pattern. As
you can see, it works nicely.
Examples
Finding out who an e-mail is from. Have you ever saved an e-mail as a text file? If you have, you may have seen
that it contains a lot of essentially unreadable text at the top, similar to that shown in Listing 10-9.
Listing 10-9. A Set of (Fictitious) E-mail Headers
From foo@bar.baz Thu Dec 20 01:22:50 2004
Return-Path:
Received: from xyzzy42.bar.com (xyzzy.bar.baz [123.456.789.42])
by frozz.bozz.floop (8.9.3/8.9.3) with ESMTP id BAA25436
for ; Thu, 20 Dec 2004 01:22:50 +0100 (MET)
Received: from [43.253.124.23] by bar.baz
(InterMail vM.4.01.03.27 201-229-121-127-20010626) with ESMTP
id ;
Thu, 20 Dec 2004 00:22:42 +0000