o_18ufhmfmq19t513t3lgmn5l1qa8a.pdf

CHAPTER 10 ■ BATTERIES INCLUDED 243
The easiest way to harness the power of re.sub is to use group numbers in the substitution
string. Any escape sequences of the form '\\n' in the replacement string are replaced by the
string matched by group n in the pattern. For example, let’s say you want to replace words of
the form '*something*' with 'something', where the former is a normal way of
expressing emphasis in plain text documents (such as e-mail), and the latter is the corresponding
HTML code (as used in Web pages). Let’s first construct the regexp:
>>> emphasis_pattern = r'\*([^\*]+)\*'
Note that regular expressions can easily become hard to read, so using meaningful variable
names (and possibly a comment or two) is important if anyone (including you!) is going to be
able to read the code.
■Tip One way to make your regular expressions more readable is to use the VERBOSE flag in the re functions.
This allows you to add whitespace (space characters, tabs, newlines, and so on) to your pattern, which
will be ignored by re—except when you put it in a character class or escape it with a backslash. You can also
put comments in such verbose regexps. The following is a pattern object that is equivalent to the emphasis
pattern, but which uses the VERBOSE flag:
>>> emphasis_pattern = re.compile(r'''
... \* # Beginning emphasis tag -- an asterisk
... ( # Begin group for capturing phrase
... [^\*]+ # Capture anything except asterisks
... ) # End group
... \* # Ending emphasis tag
... ''', re.VERBOSE)
...
Now that I have my pattern, I can use re.sub to make my substitution:
>>> re.sub(emphasis_pattern, r'\1', 'Hello, *world*!')
'Hello, world!'
As you can see, I have successfully translated the text from plain text to HTML.
But you can make your substitutions even more powerful by using a function as the
replacement. This function will be supplied with the MatchObject as its only parameter, and
the string it returns will be used as the replacement. In other words, you can do whatever you
want to the matched substring, and do elaborate processing to generate its replacement. What
possible use could you have for such power, you ask? Once you start experimenting with regular
expressions, you will surely find countless uses for this mechanism. For one application, see
the “Examples” section that follows.