Met module “re” kan een string verwerkt worden volgens een bepaalde pattern. Meestal wordt het gebruikt om mix van tekst en speciale tekens te zoeken.
. Matches any character except newline by default
^ Matches the start of the string
$ Matches the end of the string
* "Any number of occurrences of what just preceded me"
+ "One or more occurrences of what just preceded me"
| "Either the thing before me or the thing after me"
\w Matches any alphanumeric character
\d Matches any decimal digit
tomato Matches the string tomato

foo.* Matches any string starting with foo
\d* Match any number decimal digits
[a-zA-Z]+ Match a sequence of one or more letters

Voorbeeld:
class Options:
def read_db_connect(self):
f = open("db_connect.txt")
str_content = f.read()
f.close()
host = self.read_options(str_content, "host")
user = self.read_options(str_content, "user")
passwd = self.read_options(str_content, "passwd")
db = self.read_options(str_content, "db")
return host, user, passwd, db
def read_options(self, str_content, option):
pattern = re.compile("" + option + "=\"(\S*)\"")
list_option = re.findall(pattern, str_content)
return list_option[0]
Om volledig lijst te weergeven:
>>> import sre
>>> help(sre)
Help on module sre:

NAME
sre - Support for regular expressions (RE).

FILE
c:\python24\lib\sre.py

DESCRIPTION
This module provides regular expression matching operations similar to
those found in Perl. It supports both 8-bit and Unicode strings; both
the pattern and the strings being processed can contain null bytes and
characters outside the US ASCII range.
Regular expressions can contain both special and ordinary characters.
Most ordinary characters, like "A", "a", or "0", are the simplest
regular expressions; they simply match themselves. You can
concatenate ordinary characters, so last matches the string 'last'.
The special characters are:
"." Matches any character except a newline.
"^" Matches the start of the string.
"$" Matches the end of the string.
"*" Matches 0 or more (greedy) repetitions of the preceding RE.
Greedy means that it will match as many repetitions as possible.
"+" Matches 1 or more (greedy) repetitions of the preceding RE.
"?" Matches 0 or 1 (greedy) of the preceding RE.
*?,+?,?? Non-greedy versions of the previous three special characters.
{m,n} Matches from m to n repetitions of the preceding RE.
{m,n}? Non-greedy version of the above.
"\\" Either escapes special characters or signals a special sequence.
[] Indicates a set of characters.
A "^" as the first character indicates a complementing set.
"|" A|B, creates an RE that will match either A or B.
(…) Matches the RE inside the parentheses.
The contents can be retrieved or matched later in the string.
(?iLmsux) Set the I, L, M, S, U, or X flag for the RE (see below).
(?:…) Non-grouping version of regular parentheses.
(?P<name>…) The substring matched by the group is accessible by name.
(?P=name) Matches the text matched earlier by the group named name.
(?#…) A comment; ignored.
(?=…) Matches if … matches next, but doesn't consume the string.
(?!…) Matches if … doesn't match next.
The special sequences consist of "\\" and a character from the list
below. If the ordinary character is not on the list, then the
resulting RE will match the second character.
\number Matches the contents of the group of the same number.
\A Matches only at the start of the string.
\Z Matches only at the end of the string.
\b Matches the empty string, but only at the start or end of a word.
\B Matches the empty string, but not at the start or end of a word.
\d Matches any decimal digit; equivalent to the set [0-9].
\D Matches any non-digit character; equivalent to the set [^0-9].
\s Matches any whitespace character; equivalent to [ \t\n\r\f\v].
\S Matches any non-whitespace character; equiv. to [^ \t\n\r\f\v].
\w Matches any alphanumeric character; equivalent to [a-zA-Z0-9_].
With LOCALE, it will match the set [0-9_] plus characters defined
as letters for the current locale.
\W Matches the complement of \w.
\\ Matches a literal backslash.
This module exports the following functions:
match Match a regular expression pattern to the beginning of a string.
search Search a string for the presence of a pattern.
sub Substitute occurrences of a pattern found in a string.
subn Same as sub, but also return the number of substitutions made.
split Split a string by the occurrences of a pattern.
findall Find all occurrences of a pattern in a string.
compile Compile a pattern into a RegexObject.
purge Clear the regular expression cache.
escape Backslash all non-alphanumerics in a string.
Some of the functions in this module takes flags as optional parameters:
I IGNORECASE Perform case-insensitive matching.
L LOCALE Make \w, \W, \b, \B, dependent on the current locale.
M MULTILINE "^" matches the beginning of lines as well as the string.
"$" matches the end of lines as well as the string.
S DOTALL "." matches any character at all, including the newline.
X VERBOSE Ignore whitespace and comments for nicer looking RE's.
U UNICODE Make \w, \W, \b, \B, dependent on the Unicode locale.
This module also defines an exception 'error'.

CLASSES
exceptions.Exception
sre_constants.error
class error(exceptions.Exception)
| Methods inherited from exceptions.Exception:
|
| __getitem__(…)
|
| __init__(…)
|
| __str__(…)

FUNCTIONS
compile(pattern, flags=0)
Compile a regular expression pattern, returning a pattern object.
escape(pattern)
Escape all non-alphanumeric characters in pattern.
findall(pattern, string, flags=0)
Return a list of all non-overlapping matches in the string.
If one or more groups are present in the pattern, return a
list of groups; this will be a list of tuples if the pattern
has more than one group.
Empty matches are included in the result.
finditer(pattern, string, flags=0)
Return an iterator over all non-overlapping matches in the
string. For each match, the iterator returns a match object.
Empty matches are included in the result.
match(pattern, string, flags=0)
Try to apply the pattern at the start of the string, returning
a match object, or None if no match was found.
purge()
Clear the regular expression cache
search(pattern, string, flags=0)
Scan through string looking for a match to the pattern, returning
a match object, or None if no match was found.
split(pattern, string, maxsplit=0)
Split the source string by the occurrences of the pattern,
returning a list containing the resulting substrings.
sub(pattern, repl, string, count=0)
Return the string obtained by replacing the leftmost
non-overlapping occurrences of the pattern in string by the
replacement repl. repl can be either a string or a callable;
if a callable, it's passed the match object and must return
a replacement string to be used.
subn(pattern, repl, string, count=0)
Return a 2-tuple containing (new_string, number).
new_string is the string obtained by replacing the leftmost
non-overlapping occurrences of the pattern in the source
string by the replacement repl. number is the number of
substitutions that were made. repl can be either a string or a
callable; if a callable, it's passed the match object and must
return a replacement string to be used.
template(pattern, flags=0)
Compile a template pattern, returning a pattern object

DATA
DOTALL = 16
I = 2
IGNORECASE = 2
L = 4
LOCALE = 4
M = 8
MULTILINE = 8
S = 16
U = 32
UNICODE = 32
VERBOSE = 64
X = 64
__all__ = ['match', 'search', 'sub', 'subn', 'split', 'findall', 'comp…
__version__ = '2.2.1'

Match Objects
Contain information about the match itself
But it is based on a notion of "groups"
Grouping Rules
(\d+)\.(\d*)
This regular expression has 3 groups
group 0 : The entire regular expression
group 1 : The (\d+) part
group 2 : The (\d*) part
Group numbers are assigned left to right in the pattern
Obtaining match information
m.group(n) # Return text matched for group n
m.start(n) # Return starting index for group n
m.end(n) # Return ending index for group n

Matching Example
import re
r = re.compile(r’(\d+)\.(\d*)’)
m = r.match("42.37")
a = m.group(0) # Returns ’42.37’
b = m.group(1) # Returns ’42’
c = m.group(2) # Returns ’37’
print m.start(2) # Prints 3
A more complex example
# Replace URL such as http://www.python.org with a hyperlink
pat = r’(http://[\w-]+(\.[\w-]+)*((/[\w-~]*)?))’
r = re.compile(pat)
r.sub(’<a href="\\1">\\1</a>’,s) # Replace in string

Where to go from here?

Mastering Regular Expressions, by Jeffrey Friedl
Online docs
Experiment