Beginning Python - From Novice to Professional

CHAPTER 10 ■ BATTERIES INCLUDED 241
The function re.escape is a utility function used to escape all the characters in a string that
might be interpreted as a regexp operator. Use this if you have a long string with lots of these
special characters and you want to avoid typing a lot of backslashes, or if you get a string from
a user (for example, through the raw_input function) and want to use it as a part of a regexp.
Here is an example of how it works:
>>> re.escape('www.python.org')
'www\\.python\\.org'
>>> re.escape('But where is the ambiguity?')
'But\\ where\\ is\\ the\\ ambiguity\\?'
■Note In Table 10-8 you’ll notice that some of the functions have an optional parameter called flags. This
parameter can be used to change how the regular expressions are interpreted. For more information about
this, see the standard library reference, in the section about the re module at http://python.org/doc/
lib/module-re.html. The flags are described in the subsection “Module Contents.”
Match Objects and Groups
The re functions that try to match a pattern against a section of a string all return MatchObjects
when a match is found. These objects contain information about the substring that matched
the pattern. They also contain information about which parts of the pattern matched which
parts of the substring—and these “parts” are called groups.
A group is simply a subpattern that has been enclosed in parentheses. The groups are
numbered by their left parenthesis. Group zero is the entire pattern. So, in the pattern
'There (was a (wee) (cooper)) who (lived in Fyfe)'
the groups are as follows:
0 There was a wee cooper who lived in Fyfe
1 was a wee cooper
2 wee
3 cooper
4 lived in Fyfe
Typically, the groups contain special characters such as wildcards or repetition operators,
and thus you may be interested in knowing what a given group has matched. For example, in
the pattern
r'www\.(.+)\.com$'
group 0 would contain the entire string, and group 1 would contain everything between 'www.'
and '.com'. By creating patterns like this, you can extract the parts of a string that interest you.
Some of the more important methods of re match objects are described in Table 10-9.