Python RegEx

Python RegEx Module – re

re- module is a python module developed to work with general expressions. The module contains functions and other constants to deal with regular expressions in python. Let's look at a few most commonly used functions.

• findall()

The re.findall() method is one of the most commonly used methods of this module. It returns a list of all the matches found in the given string. An example is illustrated below. It returns an empty string if no match is found.

Code:

#program to find all numbers in a string

 import re

string = ‘java 2 has 3 many45 features”

pattern = ‘\d+’


result = re.findall(pattern, string)

print(result)

Output:

[‘2’, ‘3’, ’45’]

• split()

The re.split() method looks for the match and then split the string from where it matches the given text string. It will return the original string if no split is found.

Code:

#program to find all numbers in a string

 import re


string = ‘java 2 has 3 many45 features”

pattern = ‘\d+’


result = re.split(pattern, string)

print(result)

Output:

[‘java ’, ‘ has ’, ‘ many’, ‘features’]

Note: We can pass maxsplit argument to re.split method. It restricts the number of splits that occur. It returns the splited string as output. In case the value of max split is not mentioned, 0 is the default value, meaning max no. of split occurs.

Code:

#program to find all numbers in a string

 import re



#maxsplit = 1

#spliting the string only once

string = ‘java 2 has 3 many45 features”

pattern = ‘\d+’



result = re.split(pattern, string, 1)

print(result)

Output:

[‘java ’, ‘2 has 3 many45 features’]

• sub()

The re.sub() method looks for a match defined by the pattern. After it finds the match, it replaces the matched value with a predefined replace value. It returns the original string if no match is found.

Code:

#program to remove all the whitespace from the text

 import re


#multiline string

string = ‘java 2 has 3 many45 features”


#matches all the whitespace characters

pattern = ‘\s+’


#emty string

replace = ‘’

new_string = re.sub(pattern, replace, string)

print(new_string)

Output:

java2has3many45features

We can pass a parameter called count in the argument. It restricts the no. of matches to be replaced. The default value of the count is set to 0.

Code:

#program to remove all the whitespace from the text

 import re


#multiline string

string = ‘java 2 has 3 many45 features”


#matches all the whitespace characters

pattern = ‘\s+’


#emty string

replace = ‘’


#count = 1

new_string = re.sub(pattern, replace, string, 1)

print(new_string)

Output:

java2 has 3 many 45 features

• subn()

The re.subn() method is similar to re.sub() looks for match defined by the pattern. After it finds the match, it replaces the matched value with a predefined replace value. It returns a tuple with two parameters, result, and count of replaces. It returns the original string if no match is found.

Code:

#program to remove all the whitespace from the text

 import re

#multiline string

string = ‘java 2 has 3 many 45 features”




#matches all the whitespace characters

pattern = ‘\s+’




#emty string

replace = ‘’

new_string = re.subn(pattern, replace, string)

print(new_string)

Output:

(‘java2has3many45features’, 6)

• search()

The re.search() method is doing what it sounds like. It return true if it finds a match for the given pattern.

Code:

import re

#multiline string

string = ‘java 2 has 3 many 45 features”

#check if java is at the beginning of the string

pattern = ‘\Ajava’

match = re.search(pattern, string)

if match:

            print(“match successful”)

else:

            print(“match not successful”)

Output:

match successful

Match Object in re module

The match object in the re module has several methods and attributes. A few commonly used methods of match objects are:

match.group()

The group() method returns the matched portion of the text string.

Code:

import re

#multiline string

string = ‘java 256 88 has 3 many 45 features”


#three digits followed by two digit

pattern = ‘(\d{3}) (\d{2})’


match = re.search(pattern, string)


if match:

            print(“match.group()”)

else:

            print(“match not  found”)

Output:

256 88

match.start(), match.end() and match.span()

match.start() returns the index of the first match. Similiarly, match.end() returns the index of the last matched character.

On the other hand, match.span() returns a tuple of start and end index.

>>> match.start()

5

>>>match.end()

10

>>>match,span()

(5, 10)

There are several other methods, and functions in the re module. Please refer to the official documentation to know more about the re module.