Beginning Python - From Novice to Professional

236 CHAPTER 10 ■ BATTERIES INCLUDED
The Wildcard
A regexp can match more than one string, and you create such a pattern by using some special
characters. For example, the period character (dot) matches any character (except a newline),
so the regular expression '.ython' would match both the string 'python' and the string
'jython'. It would also match strings such as 'qython', '+ython', or ' ython' (in which the first
letter is a single space), but not strings such as 'cpython' or 'ython' because the period matches
a single letter, and neither two nor zero.
Because it matches “anything” (any single character except a newline), the period is called
a wildcard.
Escaping Special Characters
When you use special characters such as this, it’s important to know that you may run into
problems if you try to use them as normal characters. For example, imagine you want to match
the string 'python.org'. Do you simply use the pattern 'python.org'? You could, but that would
also match 'pythonzorg', for example, which you probably wouldn’t want. (The dot matches
any character except newline, remember?) To make a special character behave like a normal
one, you escape it, just as I demonstrated how to escape quotes in strings in Chapter 1. You
place a backslash in front of it. Thus, in this example, you would use 'python\\.org', which
would match 'python.org', and nothing else.
■Note To get a single backslash, which is required here by the re module, you need to write two backslashes
in the string—to escape it from the interpreter. Thus you have two levels of escaping here: (1) from
the interpreter, and (2) from the re module. (Actually, in some cases you can get away with using a single
backslash and have the interpreter escape it for you automatically, but don’t rely on it.) If you are tired of
doubling up backslashes, use a raw string, such as r'python\.org'.
Character Sets
Matching any character can be useful, but sometimes you want more control. You can create a
so-called character set by enclosing a substring in brackets. Such a character set will match any
of the characters it contains, so '[pj]ython' would match both 'python' and 'jython', but nothing
else. You can also use ranges, such as '[a-z]' to match any character from a to z (alphabetically),
and you can combine such ranges by putting one after another, such as '[a-zA-Z0-9]'
to match uppercase and lowercase letters and digits. (Note that the character set will match
only one such character, though.)
To invert the character set, put the character ^ first, as in '[^abc]' to match any character
except a, b, or c.