Python Regular Expressions

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What is a regular expression?
“A string that defines a text
matching pattern”
Jill roll number is 1001
Bob roll number is 1002
Rob roll number is 1003
Jack roll number is 1004 Extract Roll Numbers ?
dddd
Regular Expression
1001
1002
1003
1004

What is the advantage of using
regular expressions?
 Using regular expressions, You can extract
text which follows a pattern by writing only
very few lines of codes

Example
A weight is 46kg
B weight is 54kg
C weight is 60kg
D weight is 70kg
Text File
46
54
60
70
Extract
Without Using Regular Expressions
 Lengthy
Code
 Complex

Example
A weight is 46kg
B weight is 54kg
C weight is 60kg
D weight is 70kg
Text File
46
54
60
70
Extract
Using Regular Expressions
 Less Code
 Simple

Python String
Set of characters enclosed in single or
double quotes
Ex: ‘Kalyan’, “Meghana”

Python Raw Strings
Set of characters enclosed in single or
double quotes preceded by r
Ex: r‘Kalyan’, r“Meghana”

Strings vs Raw Strings
You can write a regular expression as a
string or raw string
In a string regular expression, you have
to escape the special characters.
In a raw string regular expression, you
need not to escape the special
characters

Regular Expressions
 Regular Expression are supported by many programming languages
Ex: Perl, Ruby , Java, Python, Javascript ……………
 Some languages provide regex capabilities built in
Ex: Perl
 Some languages provide regex capabilities via libraries
Ex: Python

re Module
Python supports regular expressions through re
module
That is, you have to import re module for
using regular expressions
import re
No need to explicitly install this module

Steps
Import re module
Write regular expression
Create regex object
Call the function using regex object

re Module Functions
 match(text)
 search(text)
 findall(text)
 finditer(text)
 sub(replString, text)
 split(text)

match()
o Looks for the match at the beginning of the string
o Returns a match object if there is a match , otherwise
returns None
regex=re.compile(pattern)
mo=regex.match(text)

search()
o Looks for the match any where in the string
o Returns a match object if there is a match , otherwise
returns None
o If string has more than one match, returns the match
object for the first match only
mo=regex.search(text)

findall()
o Returns all matched substrings as a list if there is
match, otherwise returns empty list
values=regex.findall(text)

finditer()
o Returns objects for all matched substrings as a list
if there is a match, otherwise returns empty list
moList=regex.finditer(text)

sub()
o replaces all the matched substrings with the
given replString and returns the modified
string, if there is match
o Returns original string, if there is no match
o Similar to replace option in text editors
regex.sub(replString,text)

split()
o Looks for match anywhere in the string
o Splits the string at the matched substrings and
returns the splitted string as a list
o Returns original string, if there is no match
o Similar to split() method in strings
regex.split(text)

Groups
o You want to match a substring in a string and want to
extract a part of matched substring, grouping is used.
Match the roll number CS1004 and extract the last four digits

Groups - Types
oNumbered Groups
oNamed Groups
oNon-capturing Groups

Numbered Groups

Named Groups
 When groups are
large in number,
it is difficult
to remember the
group numbers
 In such a case,
we use named
groups

NonCapturing Group(?:)

Meta Characters
| (pipe)
? (question mark)
* (asterisk)
+ (plus symbol)
. (dot symbol)

|(pipe)
Matches one of the many characters
Matches
42
100
30
111
A weight is 42kg
B weight is 100kg
C weight is 30kg
D weight is 111kg

?(question mark)
Matches zero or one occurrence
Matches
42
100
30
111
A weight is 42kg
B weight is 100kg
C weight is 30kg
D weight is 111kg

*(asterisk)
Matches zero or more occurrence
Matches
abbbc
abc
ac
abbbc
abc
ac

+(plus symbol)
Matches one or more occurrence
Matches
abbbc
abbc
abc
abbbc
abbc
abc

.(dot symbol)
Matches any character except ‘n’
Matches
Kalyan007
Kalyann007

pattern{m}
Matches exactly m repetitions
Matches exactly 3 digits

pattern{m,n}
Matches minimum of m repetitions
& maximum of n repetitions

pattern{m,}
Matches a minimum of m repetitions
.
.
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Greedy Matching
Looks for the maximum possible match
abcabcabcabc Greedy Match
abcabcabcabc

NonGreedy Matching(?)
Looks for the minimum possible match
abcabcabcabc NonGreedy Match
abc

Character Classes
Matches one of the many characters
Types
Positive Character Class
Negative Character Class
Shorthand Character Class

Positive Character Class
Matches one of the characters specified in []
[abc] Matches a or b or c
[aeiou] Matches a ,e,i,o,u
[0123456789] Matches numbers 0 to 9
[a-c0-9] Matches a,b,c or 0 to 9

Negative Character Class
Matches any character other than the characters
specified in [^]
[^aeiou] Matches other an aeiou
b1001
c1002
d1003
f1004
h1005
b1001
c1002
d1003
f1004
h1005

Shorthand Character Class
d any decimal digit [0-9]Matches Equivalent to
D Any non- digit [^0-9]Matches Equivalent to
w any word character [a-zA-z_0-9]Matches Equivalent to
W
any non word char
[^a-zA-Z_0-9]Matches Equivalent to
s any space character [ntrfv]Matches Equivalent to
S Any non space char [^ntvrf]Matches Equivalent to

Anchoring
Specify the relative location of the match
Anchoring Meaning
^ Start of line or string
$ End of line or string
A Start of string
Z End of string
b Word boundary
B Non word boundary

^ - Start of line or String
Specify the location of the match as “start of
line or string”
r’^Hello’
r’hello’
Matches hello anywhere in
the input string
Matches hello at the
beginning of input string

$ - End of line or String
Specify the location of the match as “end of
line or string”
r’bye$’
r’bye’
Matches bye anywhere in the
input string
Matches bye at the end of
input string

b – Word boundary
Specify the word boundary
Matches any character other than word characters
that is, other than [a-zA-z0-9_]
bcatb
Matches cat in
“My cat”
“Your cat”
“cat1 is good”
“(cat) is pet”
But not in
“Concatenation of strings”
“catalyst is zinc”

B – Non word boundary
Specify the non word boundary
Opposite to b
BcatB
Matches cat in
“Concatenation of strings”
“Acatalyst is zinc”
But not in
“My cat”
“Your cat”
“cat1 is good”
“(cat) is pet”

Compilation Flags
 compile() has two paramters, first one is the
pattern and second one is compilation flag
which is optional.
re.compile(pattern, [flag])
 Compilation flags can be passed to compile() or
it can be embedded in the regex pattern itself
Pattern=r’(?i)w+’

Compilation Flags
Compilation Flag Meaning
re.IGNORECASE OR re.I Case insensitive matching
(i)
re.DOTALL or re.S . Matches any character
including ‘n’ (s)
re.VERBOSE or re.X Ignores white spaces and
comments (x)
re.MULTILINE or re.M Enable multi line mode (m)
re.UNICODE or re.U Enable the Unicode mode (u)

Assertions
Look Ahead Assertions
Positive Look Ahead Assertions
Negative Look Ahead Assertions
Look Behind Assertions
Positive Look Behind Assertions
Negative Look Behind Assertions

Python Regular Expressions

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