Python Ninja Bootcamp 40-Regular expression

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In this Python lecture, we will learn about the Regular Expression.

"Regular expression is a sequence of characters that forms a search pattern

Python has a built-in package called,re module which can be used to work with Regular Expressions."

Important methods in Regex

1. match( )
Takes a pattern and string as parameter. It applies the pattern at the beginning of the string & returns the match object if found or None if not found

Let see an example-

In order to work with regular expression, we need to import re

import re

pattern='python'
string='python ninja bootcamp'

if re.match(pattern,string):

    print('Match is found')
else:
    print('No Match is found')

>>Match is found

pattern='python'

string='Welcome to python ninja bootcamp'

if re.match(pattern,string):

    print('Match is found')
else:

    print('No Match is found')

>>No Match is found

2. search( )

takes a pattern and string as a parameter. It matches the pattern anywhere in the string & returns the object if found or None if not found

pattern='python'
string='Welcome to python ninja bootcamp'

if re.search(pattern,string):
    print('Match is found')
else:
    print('No Match is found')

>>Match is found

3. findall( )

Takes a pattern and string as a parameter. It returns the list of all substrings that match a pattern.

pattern='python'
string='python111python222python333'

re.findall(pattern,string)

>> ['python', 'python', 'python']

Metacharacters

Metacharacters are the building block of a regular expression, characters having some important meaning.

Important Metacharacter

1. dot
. matches with any character, other than \n.

pattern='b.t'
string='bat_bet_bot_but_b\nt'

re.findall(pattern,string)

>> ['bat', 'bet', 'bot', 'but']

2. Asterisks
* matches with zero or more occurrence of a character.

pattern='ab*'
string='a_ab_abbb_abc_b'

re.findall(pattern,string)

>>['a', 'ab', 'abbb', 'ab']

3. Plus
+ matches with one or more occurrence of a character.

pattern='ab+'
string='a_ab_abbb_abc_b'

re.findall(pattern,string)

>> ['ab', 'abbb', 'ab']

4. Question Mark
? matches with zero or one occurrence of a character.

pattern='ab?'
string='a_ab_abbb_abc_b'

re.findall(pattern,string)

>>['a', 'ab', 'ab', 'ab']

5. Curly Bracket

{} matches with the number defined inside it.

pattern='ab{3}'
string='a_ab_abbb_abc_b'

re.findall(pattern,string)

>> ['abbb']

6. Caret
^ matches pattern at the beginning of the string.

pattern='^ab'
string='abcd'

bool(re.search(pattern,string))

>> True

7. Dollar

$ matches the pattern at the beginning of the string.

pattern='cd$'
string='abcd'

bool(re.search(pattern,string))

>> True

Character Set in Regex.

Character set matches only one out of several characters.

It is defined by putting the character in the [ ].

pattern='b[aeiou]t'

string='bat'

bool(re.search(pattern,string))

>> True

pattern='b[aeiou]t'

string='bot'

bool(re.search(pattern,string))

>> True

pattern='b[aeiou]t'
string='bxt'

bool(re.search(pattern,string))

>> False

Caret & Character set
Caret inside [ ] excludes all the character defined inside it

pattern='b[^aeiou]t'
string='bxt'

bool(re.search(pattern,string))

>> True

pattern='b[^aeiou]t'
string='bxt'

bool(re.search(pattern,string))

>> False

Range & Character Set

Describe the range of character and numbers in character set.

pattern='[A-Z][0-9]'
string='A1'

bool(re.search(pattern,string))

>> True

pattern='[A-Z][0-9]'
string='A+'

bool(re.search(pattern,string))

>> False

Some Special Sequence

\d : for digit

pattern='\d+'
string='Welcome to #Python Ninja Bootcamp 007. Lets get started'

re.findall(pattern,string)

>>['007']

\D : for non-digit

pattern='\D+'
string='Welcome to #Python Ninja Bootcamp 007. Lets get started'

re.findall(pattern,string)

>>['Welcome to #Python Ninja Bootcamp ', '. Lets get started']

\s : for white space

pattern='\s+'
string='Welcome to #Python Ninja Bootcamp 007. Lets get started'

re.findall(pattern,string)

>>[' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ']

\S : for non-white space

pattern='\S+'
string='Welcome to #Python Ninja Bootcamp 007. Lets get started'

re.findall(pattern,string)

>>['007']

\w : for alphanumeric

pattern='\w+'
string='Welcome to #Python Ninja Bootcamp 007. Lets get started'

re.findall(pattern,string)

>>['Welcome','to','#Python','Ninja','Bootcamp','007.','Lets','get','started']

\W : for non- alphanumeric

pattern='\W+'
string='Welcome to #Python Ninja Bootcamp 007. Lets get started'

re.findall(pattern,string)

>>[' ', ' #', ' ', ' ', ' ', '. ', ' ', ' ']

EDITS ARE WELCOMED!!

In the next Blog, we will discuss Q&A Seventh

https://sngurukuls247.blogspot.com/2018/12/python-ninja-bootcamp-41-q-7th.html

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