Regex in Python.md

Python regex

Regular expressions are used for pattern matching in a string. They provide a general way for matching patters for finding valid phone numbers, email addresses which adhere to certain format. This is a tedious task to do using normal search functionalities. Regular expressions are implemented internally using finite automata.

re.search(pattern, string)

matches first occurance of pattern in the string and returns match object containing index of match and the matched string

import re  #import regex module

#appending r before string makes it raw string
print(re.search('n', r'ba\n'))

running above program ouput will be :

<_sre.SRE_Match object; span=(3, 4), match='n'>

Match object methods

Method	Functionality
start()	return starting index of matched string
end()	return ending index of matched string
span()	return starting and ending index as tuple
group()	return matched string

import re 

print(re.search('ab', '12abcd').start()) 
print(re.search('ab', '12abcd').end())  
print(re.search('ab', '12abcd').span())  
print(re.search('ab', '12abcd').group())

running above program ouput will be :

2
4
(2, 4)
ab

re.match(pattern, string)

matches pattern at the beginning of the string (prefix matching) and returns the match object

import re 

print(re.match('abc', 'abcdef'))

running above program ouput will be :

<_sre.SRE_Match object; span=(0, 3), match='abc'>

re.findall(pattern, string)

finds all occurances of pattern in string and returns list of matched strings

import re 

print(re.findall('[0-9]+', '12ad123cd'))

running above program ouput will be :

['12', '123']

re.sub(pattern, repla, string)

replace all matches in string with repla and returns modified string

import re 

print(re.sub('ab', 'cd', 'habcabc'))

running above program ouput will be :

hcdccdc

re.compile(pattern)

compiles pattern into returns regular expression object. This object can be used for matching using match(), search() and other methods.

import re 

p = re.compile('ab') #p can be reused
print(p.search('cab')) 

running above program ouput will be :

<_sre.SRE_Match object; span=(1, 3), match='ab'>

Repetitions

* is used to match 0 or more occurances of preceding character

? is used to match 0 or 1 occurance of preceding character

+ is used to match 1 or more occurances of preceding character

{x,y} is used to match atleast x occurances and atmost y occurances of preceding character

import re

print(re.search('ab*c', 'abbbbbc')) 
print(re.search('ab?c', 'ac'))
print(re.search('ab+c', 'abbc'))
print(re.search('a{1,4}', 'aaabcd'))

running above program ouput will be :

<_sre.SRE_Match object; span=(0, 7), match='abbbbbc'>
<_sre.SRE_Match object; span=(0, 2), match='ac'>
<_sre.SRE_Match object; span=(0, 4), match='abbc'>
<_sre.SRE_Match object; span=(0, 3), match='aaa'>

Metacharacters

[] are used to enclose a group of characters to be matched

[0-9] can match any decimal digit

[a-d] can match a, b, c, or d

^ is used to exclude character from matching

. matches any character except newline character

import re

print(re.search('[a-c]{2}', 'dmacf')) 
print(re.search('[0-9]{3}', 'aa123'))
print(re.search('[^a-c]', 'ca1d'))
print(re.search('.+', 'aaabcd\n'))

running above program ouput will be :

<_sre.SRE_Match object; span=(2, 4), match='ac'>
<_sre.SRE_Match object; span=(2, 5), match='123'>
<_sre.SRE_Match object; span=(2, 3), match='1'>
<_sre.SRE_Match object; span=(0, 6), match='aaabcd'>

\w matches any alpha numeric character, equivalent to [a-zA-Z0-9_]

\d matches any digit, equivalent to [0-9]

\W matches character other than alpha numeric character, equivalent to [^a-zA-Z0-9_]

\D matches any non decimal character, equivalent to [^0-9]

\s matches any whitespace character

import re

print(re.search('\w\w', 'c123')) 
print(re.search('\d\d\d', 'aa123bc'))
print(re.search('\W', 'ca\n'))
print(re.search('\D', '123c23'))
print(re.search('\s', 'how are'))

running above program ouput will be :

<_sre.SRE_Match object; span=(0, 2), match='c1'>
<_sre.SRE_Match object; span=(2, 5), match='123'>
<_sre.SRE_Match object; span=(2, 3), match='\n'>
<_sre.SRE_Match object; span=(3, 4), match='c'>
<_sre.SRE_Match object; span=(3, 4), match=' '>