If we want to represent a group of Strings according to a particular format/pattern then we should go for Regular Expressions.
i.e Regualr Expressions is a declarative mechanism to represent a group of Strings accroding to particular format/pattern.
Eg 1: We can write a regular expression to represent all mobile numbers
Eg 2: We can write a regular expression to represent all mail ids.
The main important application areas of Regular Expressions are
- To develop validation frameworks/validation logic
- To develop Pattern matching applications (ctrl-f in windows, grep in UNIX etc)
- To develop Translators like compilers, interpreters etc
- To develop digital circuits
- To develop communication protocols like TCP/IP, UDP etc.
We can develop Regular Expression Based applications by using python module: re
This module contains several inbuilt functions to use Regular Expressions very easily in our applications.
1. compile()
re module contains compile() function to compile a pattern into RegexObject.
pattern = re.compile("ab")
2. finditer():
Returns an Iterator object which yields Match object for every Match
matcher = pattern.finditer("abaababa")
On Match object we can call the following methods.
- start() => Returns start index of the match
- end() => Returns end+1 index of the match
- group() => Returns the matched string
Eg:
1) import re count=0
2) pattern=re.compile("ab")
3) matcher=pattern.finditer("abaababa")
4) for match in matcher:
5) count+=1
6) print(match.start(),"...",match.end(),"...",match.group())
7) print("The number of occurrences: ",count) Output: 0 ... 2 ... ab 3 ... 5 ... ab 5 ... 7 ... ab The number of occurrences: 3
Pre defined Character classes:
\s => Space character
\S => Any character except space character
\d => Any digit from 0 to 9
\D => Any character except digit
\w => Any word character [a-zA-Z0-9]
\W => Any character except word character (Special Characters)
. => Any character including special characters
Qunatifiers:
We can use quantifiers to specify the number of occurrences to match.
a => Exactly one 'a'
a+ => Atleast one 'a'
a* => Any number of a's including zero number
a? => Atmost one 'a' ie either zero number or one number
a{m} => Exactly m number of a's
a{m,n} => Minimum m number of a's and Maximum n number of a's
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