Learning Python: Programming and Data Structures- Tutorial 13- Regular Expression Matching

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Learning Python: Programming and Data Structures- Tutorial 13- Regular Expression Matching

We'll take a quick look at regular expressions in Python.

1. We will need to 'import re' first.

2. We will create a test string and in that test string, we will try to search for a word starting with the letter 'H'.

Searching for a pattern in a string

Python 2.7.3 (default, Aug 1 2012, 05:16:07)

[GCC 4.6.3] on linux2

Type "help", "copyright", "credits" or "license" for more information.

>>> import re

>>> testString1 = "Time to say Hello World"

>>> match_object = re.search(r"(H....)",testString1)

>>> print match_object.group(1)

Output:

Hello

Searching for Multiple Instances of a Pattern in a String

Let us try to look for all the numbers which occur in this string: "This is a string with numbers in it: 1, 100, 2, 3, 4,99"

For finding all instances of a pattern in a string, we use 'findall'.

>>> testString2 = "This is a string with numbers in it: 1, 100, 2, 3, 4,99"

>>> numbersInTheString = re.findall(r"(?:^|\b)([0-9]+)(?:\b)",testString2)

>>> for matchedNumber in numbersInTheString: print matchedNumber

Output:

...

100

Replacing a pattern in a String, with a Substring

This can be done using "sub".

>>> numbersInTheString = re.sub(r"(?:^|\b)s[a-z]+","word_starting_with_s",testString2)

>>> print numbersInTheString

Output:

This is a word_starting_with_s with numbers in it: 1, 100, 2, 3, 4,99

Example: Detecting an Email Address in a chunk of text

>>> testString = "This is a string with an [email protected] embedded in it."

>>> matchObj = re.search(r"(?:\b)([^\s]*\@[^\s]*\.[a-zA-Z]{2,4})(?:\b)",testString)

>>> print matchObj.group(1)

Output:

[email protected]

Example: Detecting the domain name in a given Url

>>> testString="http://www.thehindu.com/features/education/issues/engineering-students-locked-into-microsoft-office/article4640546.ece"

>>> matchObj = re.search(r"(?:https?\:\/\/)?([^\/]*)",testString)

>> print matchObj.group(1)

Output:

www.hindu.com

Important Building Blocks of Regular Expressions and Examples

Symbol/Character/Character Class Explanation Example
. Any Character matchObj = re.search(r"(..o)","Hello")
print matchObj.group(1)

Output:
llo

Explanation: Regex picks up a group which ends with an 'o' and has any two characters before that 'o'.
* Zero or more occurances of a particular character matchObj = re.search(r"e(l*)","Hello")
print matchObj.group(1)

Output:
ll

Explanation: Picks up the two consecutive 'l'.
+ One or more instances of a particular character matchObj = re.search(r"(l+)","Hello")
print matchObj.group(1)

Output:
ll
? Zero or One occurance of a particular character. If this is used after a * or a +, it tries to do a lazy match and tries to match as few characters as possible, to fit the regex. Detecting the domain. We'd like to consider Url's starting with https: or http:.

>>> testString="http://www.thehindu.com/features/education/issues"
>>> matchObj = re.search(r"(?:https?\:\/\/)?([^\/]*)",testString)
>> print matchObj.group(1)

Output:
www.hindu.com
[a-z] or [A-Z] Matches any character between, and including, [a to z] or [A to Z] matchObj = re.search(r"([a-z]+)","Hello, world")
print matchObj.group(1)

Output:
ello
[0-9] Matches any digit between, and including [0-9] Example:

>>> testString2 = "This is a string with numbers in it: 1, 100, 2, 3, 4,99"
>>> numbersInTheString = re.findall(r"(?:^|\b)([0-9]+)(?:\b)",testString2)
>>> for matchedNumber in numbersInTheString: print matchedNumber

Output:
...
1
100
2
3
4
99
\W Matches a Non-Alpha numeric character excluding _
>>> testString = "Convert all non-alphanumeric characters in this string to blanks."
>>> print re.sub(r"[\W]","*",testString)

Output:
Convert*all*non*alphanumeric*characters*in*this*string*to*blanks*

\w Matches an alpha-numeric character including -
>>>testString = "A 100 men"
>>> print re.sub(r"[\w]","*",testString)

Output:
* *** ***
\D Matches a Non-Digit character
>>> testString = "A 100 men"
>>> print re.sub(r"[\D]","*",testString)

Output: All non-digit characters have been replaced by *
**100****
\d Matches a Digit Character
>>> testString = "A 100 men"
>>> print re.sub(r"[\d]","*",testString)

Output: All digits have been replaced by *
A *** men
$ End of a string or line
\Z End of a string
\s Whitespace
^ Beginning of a string
{m,n} Between m and n occurances of a particular character.
[^...] Matches every character other than the ones inside the box brackets.