Beginning Python - From Novice to Professional

238 CHAPTER 10 ■ BATTERIES INCLUDED
The question mark means that the subpattern can appear once or not at all. There are a
few other operators that allow you to repeat a subpattern more than once:
(pattern)*
(pattern)+
(pattern){m,n}
pattern is repeated zero or more times
pattern is repeated one or more times
pattern is repeated from m to n times
So, for example, r'w*\.python\.org' matches 'www.python.org', but also '.python.org',
'ww.python.org', and 'wwwwwww.python.org'. Similarly, r'w+\.python\.org' matches
'w.python.org' but not '.python.org', and r'w{3,4}\.python\.org' matches only
'www.python.org' and 'wwww.python.org'.
■Note The term match is used loosely here to mean that the pattern matches the entire string. The match
function, described in the text that follows, requires only that the pattern matches the beginning of the string.
The Beginning and End of a String
Until now, you’ve only been looking at a pattern matching an entire string, but you can also
try to find a substring that matches the patterns, such as the substring 'www' of the string
'www.python.org' matching the pattern 'w+'. When you’re searching for substrings like this,
it can sometimes be useful to anchor this substring either at the beginning or the end of the
full string. For example, you might want to match 'ht+p' at the beginning of a string, but
not anywhere else. Then you use a caret ('^') to mark the beginning: '^ht+p' would match
'http://python.org' (and 'htttttp://python.org', for that matter) but not 'www.http.org'.
Similarly, the end of a string may be indicated by the dollar sign ('$').
■Note For a complete listing of regexp operators, see the standard library reference, in the section “Regular
Expression Syntax” (http://python.org/doc/lib/re-syntax.html).
Contents of the re Module
Knowing how to write regular expressions isn’t much good if you can’t use them for anything.
The re module contains several useful functions for working with regular expressions. Some of
the most important ones are described in Table 10-8.