Entry

How do I extract a substring from a string?

Nov 12th, 2002 05:18
Michael Chermside, Daniela Zu,

Getting a substring can be done in numerous ways... but curiously 
enough the string module is rarely involved. The simplest way, and the 
one most often used is to use "slicing" (a range of subscripts):
   >>> s = "dog cat aardvark"
   >>> s[0:3]
   'dog'
That returned a new string containing everything starting with 
character 0, up until just before the character 3.
Often, you won't know exactly what indexes to use, and you will 
calculate them with a formula. Here, for instance, we search for all 
text from the first letter 'c' to the first letter 't' (adding 1 to 
INCLUDE the 't' in our substring):
   >>> s[ s.index('c') : s.index('t')+1 ]
   'cat'
If you know that you want the beginning or end of a string, then just 
ommit that index. Also, you can measure backwards from the end of the 
string by using negative numbers. For instance, this creates a 
substring of the last 3 characters of the string:
   >>> s[-3: ]
   'ark'
All of those techniques are ones which will work on ANY sequence 
(lists, tuples, and other things in addition to strings). However, 
there's one VERY powerful technique which is limited to strings. 
Substrings can be selected using regular expressions by importing the 
re module:
   >>> import re
   >>> re.search('aa[a-z]*', s).group()
   'aardvark'
This module can also be used to perform more powerful transformations 
than simple substrings... look at its documentation for more details.