Python_Regular Expression

Programming with Sikander
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Programming with Sikander : Python: Regular Expression
 Regular expressions are a powerful tool
for various kinds of string manipulation.
 They are a domain specific language
(DSL) that is present as a library in most
modern programming languages, not just
Python.
 They are useful for two main tasks:
 identify whether a pattern exists in a given
sequence of characters (string) or not.
 performing substitutions in a string.

 Implemented in Python with the “re” module
 import re

 re.match
 re.search
 re.findall
 re.finditer
 re.fullmatch

 re.match function can be used to determine
whether pattern matches at the beginning of
a string.
 If it does, match returns an object
representing the match, if not, it returns
None.
 re.match(pattern, sequence)

 The function re.search finds a match of a
pattern anywhere in the string.

 The search function returns an object with
several methods that give details about it.
These methods include
 group which returns the string matched.
 start and end which return the start and
ending positions of the first match
 span which returns the start and end
positions of the first match as a tuple.

 The function re.findall returns a list of all
substrings that match a pattern.

 re.finditer(pattern, string)
 Return an iterator yielding match objects over all
non-overlapping matches for the pattern in string.
 The string is scanned left-to-right, and matches are
returned in the order found.

Example
Description
Character
"[a-m]"
A set of characters
[]
"d"
Signals a special sequence (can also be used to
escape special characters)

"he..o"
Any character (except newline character)
.
"^hello"
Starts with
^
"world$"
Ends with
$
"aix*"
Zero or more occurrences
*
"aix+"
One or more occurrences
+
"al{2}"
Excactly the specified number of occurrences
{}
"falls|stays"
Either or
|
Capture and group
()

Print all the vowels and its index
Input : sikander
Output :
1 i
3 a
6 e

Matches any decimal digit; this is equivalent to the class [0-9].
d
Matches any non-digit character; this is equivalent to the class [^0-9].
D
Matches any whitespace character; this is equivalent to the class [ tnr].
s
Matches any non-whitespace character; this is equivalent to the
class [^ tnr].
S
Matches any alphanumeric character; this is equivalent to the class [a-
zA-Z0-9_].
w
Matches any non-alphanumeric character; this is equivalent to the
class [^a-zA-Z0-9_].
W

 The expression d matches any digit [0-9]
 The expression D matches any character that is
not a digit.
 Given a String extract All the digits and non-digits.
Output
Input
Digits : 1 0 2 0 2 3
Non Digits : R V E C
1RV02EC023
Digits : 1 2 3 4
Non Digits : A B C D E F
ABCDE1234F
Digits : 2 0 0 0
Non Digits : R u p e e s
Rupees 2000

• s matches any whitespace character [ tnr]
• S matches any non-white space character.
 Given a String extract all spaces and Non
space characters.

• The expression w will match any word
character.
• Word characters include alphanumeric
characters (a-z,A-Z, 0-9) and underscore(_)
• Given a string, extract all word and non-word
characters (remove all special characters)

 The ^ symbol matches the position at the
start of a string.
 The $ symbol matches the position at the
end of a string.

• One or more occurrences(+) of digits

• You are given a list of phone numbers and you are
required to check whether they are valid mobile
numbers.
• A valid mobile number is a ten digit number starting
with a 7, 8 or 9.

• Verify if the given PAN number is correct.
• PAN Number:
 It’s a 10 letter string
 First 5 characters are alphabets
 Next 4 characters are digits
 Last character is alphabet

Verify if the given USN number is correct.

• Pattern = “word1|word2|word3”
• Verify if the sequence contains “from” or “to”

A group() expression returns one or more
subgroups of the match.
A groups() expression returns a tuple
containing all the subgroups of the match.

 Given an email-id seperate the username,
website and extension

A groupdict() expression returns a dictionary
containing all the named subgroups of the match,
keyed by the subgroup name.

 re.sub (pattern, repl, string)
 Returns the string obtained by replacing
the pattern in string by the replacement
repl.

Bangalore is the capital of Karnataka.
The Silicon City of India is Bangalore.
Bangalore was called garden city because of its greenary.
Task: Replace all the occurance of Bangalore to Bengaluru.

 re.compile(pattern, flags=0)
 Compile a regular expression pattern into a
regular expression object, which can be used for
matching using its match(), search() and other
methods.
 It also helps to search a pattern again without
rewriting it.

Python_Regular Expression

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