Python regular expressions

PYTHON APPLICATION
PROGRAMMING -18EC646
MODULE-3
REGULAR EXPRESSIONS
PROF. KRISHNANANDA L
DEPARTMEN T OF ECE
GSKSJTI, BENGALURU

WHAT IS MEANT BY
REGULAR EXPRESSION?
We have seen string/file slicing, searching, parsing etc and
built-in methods like split, find etc.
This task of searching and extracting finds applications in
Email classification, Web searching etc.
Python has a very powerful library called regularexpressions
that handles many of these tasks quite elegantly
Regular expressions are like small but powerful programming
language, for matching text patterns and provide a
standardized way of searching, replacing, and parsing text
with complex patterns of characters.
Regular expressions can be defined as the sequence of
characters which are used to search for a pattern in a string.
2

FEATURES OF REGEX
Hundreds of lines of code could be reduced to few lines with regular
expressions
Used to construct compilers, interpreters and text editors
Used to search and match text patterns
The power of the regular expressions comes when we add special
characters to the search string that allow us to do sophisticated
matching and extraction with very little code.
Used to validate text data formats especially input data
ARegular Expression (or Regex) is a pattern (or filter) that describes
a set of strings that matches the pattern. A regex consists of a
sequence of characters, metacharacters (such as . , d , ?, W etc ) and
operators (such as + , * , ? , | , ^ ).
Popular programming languages like Python, Perl, JavaScript, Ruby,
Tcl, C# etc have Regex capabilities 3

GENERAL USES OF REGULAR
EXPRESSIONS
Search a string (search and match)
Replace parts of a string(sub)
Break string into small pieces(split)
Finding a string (findall)
The module re provides the support to use regex in the
python program. The re module throws an exception if there
is some error while using the regular expression.
Before using the regular expressions in program, we have to
import the library using “import re”
4

REGEX FUNCTIONS
The re module offers a set of functions
FUNCTION DESCRIPTION
findall Returns a list containing all matches of a pattern in
the string
search Returns a match Object if there is a match
anywhere in the string
split Returns a list where the string has been split at each
match
sub Replaces one or more matches in a string
(substitute with another string)
match This method matches the regex pattern in the string
with the optional flag. It returns true if a match is
found in the string, otherwise it returns false.
5

EXAMPLE PROGRAM
• We open the file, loop through
each line, and use the regular
expression search() to only print
out lines that contain the string
“hello”. (same can be done using
“line.find()” also)
# Search for lines that contain ‘hello'
import re
fp = open('d:/18ec646/demo1.txt')
for line in fp:
line = line.rstrip()
if re.search('hello', line):
print(line)
Output:
hello and welcome to python class
hello how are you?
# Search for lines that contain ‘hello'
import re
for line in fp:
if re.search('hello', line):
print(line)
Output:
friends,hello and welcome
hello,goodmorning 6

EXAMPLE PROGRAM
• To get the optimum performance from Regex, we need to use special
characters called ‘metacharacters’
# Search for lines that starts with 'hello'
import re
for line in fp:
if re.search('^hello', line): ## note 'caret' metacharacter
print(line) ## before hello
Output:
hello how are you?
# Search for lines that starts with 'hello'
import re
for line in fp:
if re.search('^hello', line): ## note 'caret' metacharacter
print(line) ## before hello
Output:
hello, goodmorning
7

METACHARACTERS
Metacharacters are characters that are interpreted in a
special way by a RegEx engine.
Metacharacters are very helpful for parsing/extraction
from the given file/string
Metacharacters allow us to build more powerful regular
expressions.
Table-1 provides a summary of metacharacters and their
meaning in RegEx
Here's a list of metacharacters:
[ ] . ^ $ * + ? { } ( ) |
8

Metacharacter Description Example
[ ] It represents the set of characters. "[a-z]"
It represents the special sequence (can also be
used to escape special characters)
"r"
. It signals that any character is present at some
specific place (except newline character)
"Ja...v."
^ It represents the pattern present at the beginning
of the string (indicates “startswith”)
"^python"
$ It represents the pattern present at the end of the
string. (indicates “endswith”)
"world"
* It represents zero or more occurrences of a
pattern in the string.
"hello*"
+ It represents one or more occurrences of a
pattern in the string.
"hello+"
{} The specified number of occurrences of a pattern
the string.
“hello{2}"
| It represents either this or the other character is
present.
"hello|hi"
() Capture and group
9

[ ] - SQUARE BRACKETS
• Square brackets specifies a set of characters you wish to match.
• A set is a group of characters given inside a pair of square brackets. It represents
the special meaning.
10
[abc] Returns a match if the string contains any of the specified
characters in the set.
[a-n] Returns a match if the string contains any of the characters between a to
n.
[^arn] Returns a match if the string contains the characters except a, r, and n.
[0123] Returns a match if the string contains any of the specified digits.
[0-9] Returns a match if the string contains any digit between 0 and 9.
[0-5][0-9] Returns a match if the string contains any digit between 00 and 59.
[a-zA-Z] Returns a match if the string contains any alphabet (lower-case or upper-
case).

CONTD..
### illustrating square brackets
import re
fh = open('d:/18ec646/demo5.txt')
for line in fh:
if re.search("[w]", line):
print(line)
## search all the lines where w is
present and display
Output:
Hello and welcome
@abhishek,how are you
import re
for line in fh:
if re.search("[ge]", line):
print(line)
### Search for characters g or e or
both and display
Output:
Hello and welcome
This is Bangalore
11

CONTD…
import re
for line in fh:
if re.search("[th]", line):
print(line)
Ouput:
This is Bangalore
This is Paris
This is London
import re
for line in fh:
if re.search("[y]", line):
print(line) Ouput:
johny johny yes papa
open your mouth
### illustratingsquare brackets
import re
fh =
open('d:/18ec646/demo5.txt')
for line in fh:
if re.search("[x-z]", line):
print(line)
Output:
to:abhishek@yahoo.com
12

. PERIOD (DOT)
A period matches any single character (except newline 'n‘)
Expression String Matched?
..
(any two
characters)
a No match
ac 1 match
acd 1 match
acde
2 matches
(contains 4
characters)
### illustrating dot metacharacter
import re
for line in fh:
if re.search("y.", line):
print(line)
Output:
to: abhishek@yahoo.com
13

CONTD..
import re
for line in fh:
if re.search("P.", line):
print(line)
Output:
This is Paris
import re
for line in fh:
if re.search("T..s", line):
print(line)
Output:
This is London
These are beautiful flowers
Thus we see the great London bridge
import re
for line in fh:
if re.search("L..d", line):
print(line)
Output:
This is London
## any two characters betweenT and s
14

^ - CARET
The caret symbol ^ is used to check if a string starts with a certain
character
^a
a 1 match
abc 1 match
bac No match
^ab
abc 1 match
acb No match (starts with a but not followedby b)
### illustrating caret
import re
for line in fh:
if re.search("^h",line):
print(line) Output:
hello, goodmorning
### illustrating caret
import re
for line in fh:
if re.search("^f", line):
print(line)
from:krishna.sksj@gmail.com
15

$ - DOLLAR
The dollar symbol $ is used to check if a string ends with a certain
character.
a$
a 1 match
formula 1 match
cab No match
### illustrating metacharacters
import re
for line in fh:
if re.search("m$", line):
print(line)
Output:
import re
for line in fh:
if re.search("papa$", line):
print(line)
Output:
eating sugar no papa
16

* - STAR
The star symbol * matches zero or more occurrences of the pattern left
to it.
ma*n
mn 1 match
man 1 match
maaan 1 match
main No match (a is not followedby n)
import re
for line in fh:
if re.search("London*",line):
print(line)
Output:
This is London
17

+ - PLUS
The plus symbol + matchesone or more occurrences of the pattern left
to it.
ma+n
mn No match (no a character)
man 1 match
maaan 1 match
main No match (a is not followedby n)
import re
for line in fh:
if re.search("see+", line):
print(line)
Output:
import re
for line in fh:
if re.search("ar+", line):
print(line)
Output:
These are beautiful flowers
18

? - QUESTION MARK
The question mark symbol ? matches zero or one occurrence of the pattern left to
it.
ma?n
mn 1 match
man 1 match
maaan No match (more than one a character)
import re
for line in fh:
if re.search("@gmail?", line):
print(line)
Output:
import re
for line in fh:
if re.search("you?",line):
print(line)
Output:
19

{} - BRACES
Finds the specified number of occurrences of a pattern. Consider {n, m}. This
means at least n, and at most m repetitions of the pattern left to it.
If a{2} was given, a should be repeated exactly twice
a{2,3}
abc dat No match
abc daat 1 match (at daat)
aabc daaat 2 matches (at aabc and daaat)
aabc daaaat 2 matches (at aabc and daaaat)
20

| - ALTERNATION
Vertical bar | is used for alternation (or operator).
a|b
cde No match
ade 1 match (match at ade)
acdbea 3 matches (at acdbea)
import re
for line in fh:
if re.search("yes|no", line):
print(line)
Output:
eating sugar no papa
import re
for line in fh:
if re.search("hello|how", line):
print(line)
Output:
hello,goodmorning
21

() - GROUP
Parentheses () is used to group sub-patterns.
For ex, (a|b|c)xz match any string that matches
either a or b or c followed by xz
(a|b|c)xz
ab xz No match
abxz 1 match (match at abxz)
axz cabxz 2 matches (at axzbc cabxz)
import re
for line in fh:
if re.search("(hello|how) are", line):
print(line)
Output:@abhishek,how are you
import re
for line in fh:
if re.search("(hello and)", line):
print(line)
Ouptut:
22

- BACKSLASH
Backlash is used to escape various characters including all
metacharacters.
For ex, $a match if a string contains $ followed by a.
Here, $ is not interpreted by a RegEx engine in a special way.
If you are unsure if a character has special meaning or not, you
can put in front of it. This makes sure the character is not treated
in a special way.
NOTE :- Another way of doing it is putting the special
character in the square brackets [ ]
23

SPECIAL SEQUENCES
A special sequence is a followed by one of the characters
(see Table) and has a special meaning
Special sequences make commonly used patterns easier to
write.
24

SPECIAL SEQUENCES
Character Description Example
A It returns a match if the specified characters are
present at the beginning of the string.
"AThe"
b It returns a match if the specified characters are
present at the beginning or the end of the string.
r"bain"
r"ainb"
B It returns a match if the specified characters are
present at the beginning of the string but not at the
end.
r"Bain"
r"ainB
d It returns a match if the string contains digits [0-9]. "d"
D It returns a match if the string doesn't contain the
digits [0-9].
"D"
s It returns a match if the string contains any white
space character.
"s"
S It returns a match if the string doesn't contain any
white space character.
"S"
w It returns a match if the string contains any word
characters (Ato Z, a to z, 0 to 9 and underscore)
"w"
W It returns a match if the string doesn't contain any
word characters
"W" 25

A - Matches if the specified characters are at the start of a string.
Athe
the sun Match
In the sun No match
26
b - Matches if the specified characters are at the beginning or end of a word
bfoo
football Match
a football Match
afootball No match
foob
football No Match
the afoo test Match
the afootest No match

B - Opposite of b. Matches if the specified characters
are not at the beginning or end of a word.
Bfoo
football No match
a football No match
afootball Match
fooB
the foo No match
the afoo test No match
the afootest Match
27

d - Matches any decimal digit. Equivalent to [0-9]
D - Matches any non-decimal digit. Equivalent to [^0-9]
d
12abc3 3 matches (at 12abc3)
Python No match
D
1ab34"50 3 matches (at 1ab34"50)
1345 No match
28

s - Matches where a string contains any whitespace
character. Equivalent to [ tnrfv].
S - Matches where a string contains any non-whitespace
character. Equivalent to [^ tnrfv].
s
Python RegEx 1 match
PythonRegEx No match
S
a b 2 matches (at a b)
No match
29

w - Matches any alphanumeric character. Equivalent to [a-zA-Z0-
9_]. Underscore is also considered an alphanumeric character
W - Matches any non-alphanumeric character. Equivalent
to [^a-zA-Z0-9_]
w
12&":;c 3 matches (at 12&":;c)
%"> ! No match
W
1a2%c 1 match (at 1a2%c)
Python No match
30

Z - Matches if the specified characters are at the end of a
string.
PythonZ
I like Python 1 match
I like Python
Programming
No match
Python is fun. No match
31
# check whether the specified
#characters are at the end of string
import re
for x in fp:
x = x.rstrip()
if re.findall ("comZ", x):
print(x)
Output:

REGEX FUNCTIONS
The re module offers a set of functions
FUNCTION DESCRIPTION
findall Returns a list containing all matches of a pattern in
the string
search Returns a match Object if there is a match
anywhere in the string
split Returns a list where the string has been split at each
match
sub Replaces one or more matches in a string
(substitute with another string)
match This method matches the regex pattern in the string
with the optional flag. It returns true if a match is
found in the string, otherwise it returns false.
32

THE FINDALL() FUNCTION
The findall() function returns a list containing all matches.
The list contains the matches in the order they are found.
If no matches are found, an empty list is returned
Here is the syntax for this function −
re. findall(pattern, string, flags=0)
33
import re
str ="How are you. How is everything?"
matches= re.findall("How",str)
print(matches)
['How','How']

CONTD..
35
#check whether string starts with How
import re
str ="How are you. How is everything?"
x= re.findall("^How",str)
print (str)
print(x)
if x:
print ("string starts with 'How' ")
else:
print ("string does not start with 'How'")
Output:
How are you.How is everything?
['How']
string starts with 'How'

CONTD…
36
# match all lines that starts with 'hello'
import re
for x in fp:
x = x.rstrip()
if re.findall ('^hello',x): ## note 'caret'
print(x)
Output:
hello how are you?
# match all lines that starts with ‘@'
import re
for x in fp:
x = x.rstrip()
if re.findall ('^@',x): ## note 'caret'
metacharacter
print(x)
Output:
# check whether the string contains
## non-digit characters
import re
for x in fp:
x = x.rstrip()
if re.findall ("D", x): ## special sequence
print(x)
to:abhishek@yahoo.com
Hello and welcome

THE SEARCH() FUNCTION
The search() function searches the string for a match, and
returns a Match object if there is a match.
If there is more than one match, only the first occurrence
of the match will be returned
If no matches are found, the value None is returned
re.search(pattern, string, flags=0)
37

EXAPLES on search() function:-
outputs:
38

THE SPLIT() FUNCTION
The re.split method splits the string where there is a match
and returns a list of strings where the splits have occurred.
You can pass maxsplit argument to the re.split() method. It's
the maximum number of splits that will occur.
If the pattern is not found, re.split() returns a list containing
the original string.
re.split(pattern, string, maxsplit=0, flags=0)
39

EXAPLES on split() function:-
40
# split function
import re
for x in fp:
x = x.rstrip()
x= re.split("@",x)
print(x)
Output:
['from:krishna.sksj','gmail.com']
['to: abhishek','yahoo.com']
['Hello and welcome']
['','abhishek,how are you']

CONTD..
41
# split function
import re
fp =
for x in fp:
x = x.rstrip()
x= re.split("e",x)
print(x)
Output:
['johny johny y','s papa']
['', 'ating sugar no papa']
['t','lling li', 's']
['op','n your mouth']
Output:
['johny johny yes ', '']
['eating sugar no ','']
['telling lies']
['open your mouth']
# split function
import re
fp =
for x in fp:
x = x.rstrip()
x= re.split("papa",x)
print(x)
# split function
import re
fp =
for x in fp:
x = x.rstrip()
x= re.split("is",x)
print(x)
Output:
['Hello and welcome']
['Th',' ',' Bangalore']
['Th',' ',' Par','']
['Th',' ',' London']

THE SUB() FUNCTION
The sub() function replaces the matches with the text of your
choice
You can control the number of replacements by specifying
the count parameter
If the pattern is not found, re.sub() returns the original string
re.sub(pattern, repl, string, count=0, flags=0)
42

EXAPLES on sub() function:-
43
### illustration of substitute (replace)
import re
str ="How are you.How is everything?"
x= re.sub("How","where",str)
print(x)
Output:
where are you.where is everything?
# sub function
import re
for x in fp:
x = x.rstrip()
x= re.sub("This","Where",x)
print(x)
Output:
Hello and welcome
Where is Bangalore
Where is Paris
Where is London

THE MATCH() FUNCTION
If zero or more characters at the beginning of string match
this regular expression, return a corresponding match object.
Return None if the string does not match the pattern.
Pattern.match(string[, pos[, endpos]])
The optional pos and endpos parameters have the same
meaning as for the search() method.
44

search() Vs match()
Python offers two different primitive operations based on
regular expressions:
 re.match() checksfor a match only at the beginning of the string,
while re.search() checks for a match anywhere in the string
Eg:-
45
# match function
import re
for x in fp:
x = x.rstrip()
if re.match("This",x):
print(x)
Outptut:
This is Bangalore
This is Paris
This is London

MATCH OBJECT
A Match Object is an object containing information about the
search and the result
If there is no match, the value None will be returned, instead
of the Match Object
Some of the commonly used methods and attributes of match
objects are:
match.group(), match.start(), match.end(), match.span(),
match.string
46

match.group()
The group() method returns the part of the string where
there is a match
match.start(), match.end()
The start() function returns the index of the start of the
matched substring.
 Similarly, end() returns the end index of the matched
substring.
match.string
string attribute returns the passed string.
47

match.span()
The span() function returns a tuple containing start
and end index of the matched part.
Eg:-
OUTPUT:
(12,17)
48

Python regular expressions

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