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Friday, May 30, 2008

String Manipulation in Python

Programmers need to know how to manipulate strings for a variety of purposes, regardless of the programming language they are working in. This article will explain the various methods used to manipulate strings in Python.Introduction

String manipulation is very useful and very widely used in every language. Often, programmers are required to break down strings and examine them closely. For example, in my articles on IRC ( and, I used the split method to break down commands to make working with them easier. In my article on sockets (, I used regular expressions to look through a website and extract a currency exchange rate.

This article will take a look at the various methods of manipulating strings, covering things from basic methods to regular expressions in Python. String manipulation is a skill that every Python programmer should be familiar with.

String Methods

The most basic way to manipulate strings is through the methods that are build into them. We can perform a limited number of tasks to strings through these methods. Open up the Python interactive interpreter. Let's create a string and play around with it a bit.

>>> test = 'This is just a simple string.'

Let's take a fast detour and use the len function. It can be used to find the length of a string. I'm not sure why it's a function rather than a method, but that's a whole nother issue:

>>> len ( test )

All right, now let's get back to those methods I was talking about. Let's take our string and replace a word using the replace method:

>>> test = test.replace ( 'simple', 'short' )
>>> testa
'This is just a short string.'

Now let's count the number of times a given word, or, in this case, character, appears in a string:

>>> test.count ( 'r' )

We can find characters or words, too:

>>> test.find ( 'r' )
>>> test [ 18 ]

String Manipulation - Splitting strings, making cases

Splitting a string is something I find myself doing often. The split method is used for this:

>>> test.split()
['This', 'is', 'just', 'a', 'short', 'string.']

We can choose the point that we split it at:

>>> test.split ( 'a' )
['This is just ', ' short string.']

Rejoining our split string can be done using the join method:

>>> ' some '.join ( test.split ( 'a' ) )
'This is just some short string.'

We can play around with the case of letters in our string, too. Let's make it all upper case:

>>> test.upper()

Now let's make it lowercase:

>>> test.lower()
'this is just a short string.'

Let's capitalize only the first letter of the lowercase string:

>>> test.lower().capitalize()
'This is just a short string.'

We can also use the title method. This capitalizes the first letter in each word:

>>> test.title()
'This Is Just A Short String.'

Trading case is possible:

>>> test.swapcase()

We can run a number of tests on strings using a few methods. Let's check to see whether a given string is all upper case:

>>> 'UPPER'.isupper()
>>> 'UpPEr'.isupper()

Likewise, we can check to see whether a string contains only lower case characters:

>>> 'lower'.islower()
>>> 'Lower'.islower()

Checking whether a string looks like a title is simple, too:

>>> 'This Is A Title'.istitle()
>>> 'This is A title'.istitle()

String Manipulation - Numbers and spaces

We can check whether a string is alphanumeric:

>>> 'aa44'.isalnum()
>>> 'a$44'.isalnum()

It is also possible to check whether a string contains only letters:

>>> 'letters'.isalpha()
>>> 'letters4'.isalpha()

Here's how you check whether a string contains only numbers:

>>> '306090'.isdigit()
>>> '30-60-90 Triangle'.isdigit()

We can also check whether a string only contains spaces:

>>> ' '.isspace()
>>> ''.isspace()

Speaking of spaces, we can add spaces on either side of a string. Let's add spaces to the right of a string:

>>> 'A string.'.ljust ( 15 )
'A string. '

To add spaces to the left of a string, the rjust method is used:

>>> 'A string.'.rjust ( 15 )
' A string.'

The center method is used to center a string in spaces:

>>> 'A string.'.center ( 15 )
' A string. '

We can strip spaces on either side of a string:

>>> 'String.'.rjust ( 15 ).strip()
>>> 'String.'.ljust ( 15 ).rstrip()

String Manipulation - Regular Expressions

Regular expressions are a very powerful tool in any language. They allow patterns to be matched against strings. Actions such as replacement can be performed on the string if the regular expression pattern matches. Python's module for regular expressions is the re module. Open the Python interactive interpreter, and let's take a closer look at regular expressions and the re module:

>>> import re

Let's create a simple string we can use to play around with:

>>> test = 'This is for testing regular expressions in Python.'

I spoke of matching special patterns with regular expressions, but let's start with matching a simple string just to get used to regular expressions. There are two methods for matching patterns in strings in the re module: search and match. Let's take a look at search first. It works like this:

>>> result = ( 'This', test )

We can extract the results using the group method:

>>> ( 0 )

You're probably wondering about the group method right now and why we pass zero to it. It's simple, and I'll explain. You see, patterns can be organized into groups, like this:

>>> result = ( '(Th)(is)', test )

There are two groups surrounded by parenthesis. We can extract them using the group method:

>>> ( 1 )
>>> ( 2 )

Passing zero to the method returns both of the groups:

>>> ( 0 )

The benefit of groups will become more clear once we work our way into actual patterns. First, though, let's take a look at the match function. It works similarly, but there is a crucial difference:

>>> result = re.match ( 'This', test )
>>> print result
>>> print ( 0 )
>>> result = re.match ( 'regular', test )
>>> print result

Notice that None was returned, even though “regular” is in the string. If you haven't figured it out, the match method matches patterns at the beginning of the string, and the search function examines the whole string. You might be wondering if it's possible, then, to make the match method match “regular,” since it's not at the beginning of the string. The answer is yes. It's possible to match it, and that brings us into patterns.

The character “.” will match any character. We can get the match method to match “regular” by putting a period for every letter before it. Let's split this up into two groups as well. One will contain the periods, and one will contain “regular”:

>>> result = re.match ( '(....................)(regular)', test )
>>> ( 0 )
'This is for testing regular'
>>> ( 1 )
'This is for testing '
>>> ( 2 )

Aha! We matched it! However, it's ridiculous to have to type in all those periods. The good news is that we don't have to do that. Take a look at this and remember that there are twenty characters before “regular”:

>>> result = re.match ( '(.{20})(regular)', test )
>>> ( 0 )
'This is for testing regular'
>>> ( 1 )
'This is for testing '
>>> ( 2 )

That's a lot easier. Now let's look at a few more patterns. Here's how you can use brackets in a more advanced way:

>>> result = re.match ( '(.{10,20})(regular)', test )
>>> ( 0 )
'This is for testing regular'
>>> result = re.match ( '(.{10,20})(testing)', test )
'This is for testing'

By entering two arguments, so to speak, you can match any number of characters in a range. In this case, that range is 10-20. Sometimes, however, this can cause undesired behavior. Take a look at this string:

>>> anotherTest = 'a cat, a dog, a goat, a person'

Let's match a range of characters:

>>> result = re.match ( '(.{5,20})(,)', anotherTest )
>>> ( 1 )
'a cat, a dog, a goat'

What if we only want “a cat” though? This can be done with appending “?” to the end of the brackets:

>>> result = re.match ( '(.{5,20}?)(,)', anotherTest )
>>> ( 1 )
'a cat'

Appending a question mark to something makes it match as few characters as possible. A question mark that does that, though, is not to be confused with this pattern:

>>> anotherTest = '012345'
>>> result = re.match ( '01?', anotherTest )
>>> ( 0 )
>>> result = re.match ( '0123456?', anotherTest )
>>> ( 0 )

As you can see with the example, the character before a question mark is optional. Next is the “*” pattern. It matches one or more of the characters it follows, like this:

>>> anotherTest = 'Just a silly string.'
>>> result = re.match ( '(.*)(a)(.*)(string)', anotherTest )
>>> ( 0 )
'Just a silly string'

However, take a look at this:

>>> anotherTest = 'Just a silly string. A very silly string.'
>>> result = re.match ( '(.*)(a)(.*)(string)', anotherTest )
>>> ( 0 )
'Just a silly string. A very silly string'

What if, however, we want to only match the first sentence? If you've been following along closely, you'll know that “?” will, again, do the trick:

>>> result = re.match ( '(.*?)(a)(.*?)(string)', anotherTest )
>>> ( 0 )
'Just a silly string'

As I mentioned earlier, though, “*” doesn't have to match anything:

>>> result = re.match ( '(.*?)(01)', anotherTest )
>>> ( 0 )

What if we want to skip past the first two characters? This is possible by using “+”, which is similar to “*”, except that it matches at least one character:

>>> result = re.match ( '(.+?)(01)', anotherTest )
>>> ( 0 )

We can also match a range of characters. For example, we can match only the first four letters of the alphabet:

>>> anotherTest = 'a101'
>>> result = re.match ( '[a-d]', anotherTest )
>>> print result
>>> anotherTest = 'q101'
>>> result = re.match ( '[a-d]', anotherTest )
>>> print result

We can also match one of a few patterns using “|”::

>>> testA = 'a'
>>> testB = 'b'
>>> result = re.match ( '(a|b)', testA )
>>> print result
>>> result = re.match ( '(a|b)', testB )
>>> print result

Finally, there are a number of special sequences. “\A” matches at the start of a string. “\Z” matches at the end of a string. “\d” matches a digit. “\D” matches anything but a digit. “\s” matches whitespace. “\S” matches anything but whitespace.

We can name our groups:

>>> nameTest = 'hot sauce'
>>> result = re.match ( '(?Phot)', nameTest )
>>> ( 'one' )

We can compile patterns to use them multiple times with the re module, too:

>>> ourPattern = re.compile ( '(.*?)(the)' )
>>> testString = 'This is the dog and the cat.'
>>> result = ourPattern.match ( testString )
>>> ( 0 )
'This is the'

Of course, you can do more than match and extract substrings. You can replace things, too:

>>> someString = 'I have a dream.'
>>> re.sub ( 'dream', 'dog', someString )
'I have a dog.'

On a final note, you should not use regular expressions to match or replace simple strings.


Now you have a basic knowledge of string manipulation in Python behind you. As I explained at the very beginning of the article, string manipulation is necessary to many applications, both large and small. It is used frequently, and a basic knowledge of it is critical.

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Shahab said...

Thansk for sharing this. helped me in my assignment when I needed to learn Python very fast.

Anonymous said...

What if I want to match my string word by word not character by character because if I am matching as..
Text: This is a cat
SUBSTRING: is'(.*)is(\.*)',text,re.I)
print result

its matching "is" with Th"is" not the actual "is". Can you plz answer it where I am doing it wrong??

Dipanwita said...

keep up the work. python training in Chennai