Regular expression in python for students

Introduction to
regular expressions
REGULAR EX P RES S ION S IN P YTH ON
Maria Eugenia Inzaugarat
Data Scientist

REGULAR EXPRESSIONSIN PYTHON
What is a regular expression?
REGular EXpression or regex:
String containing a combination of normal characters and special metacharacters that describes patterns to
nd text or positions within a text

Normal characters match themselves ( st )

Metacharacters represent types of characters ( d , s , w ) or ideas ( {3,10} )

Pattern: a sequence of characters that maps to words or punctuation

Pattern matching usage:
Find and replace text
Validate strings
Very powerful and fast

The re module
import re
Find all matches of a pattern:
re.findall(r"#movies", "Love #movies! I had fun yesterday going to the #movies")
['#movies', '#movies']

The re module
import re
Split string at each match:
re.split(r"!", "Nice Place to eat! I'll come back! Excellent meat!")
['Nice Place to eat', " I'll come back", ' Excellent meat', '']

The re module
import re
Replace one or many matches with a string:
re.sub(r"yellow", "nice", "I have a yellow car and a yellow house in a yellow neighborhood")
'I have a nice car and a nice house in a nice neighborhood'

Supported metacharacters
re.findall(r"Userd", "The winners are: User9, UserN, User8")
['User9', 'User8']
re.findall(r"UserD", "The winners are: User9, UserN, User8")
['UserN']

re.findall(r"Userw", "The winners are: User9, UserN, User8")
['User9', 'UserN', 'User8']
re.findall(r"Wd", "This skirt is on sale, only $5 today!")
['$5']

re.findall(r"DatasScience", "I enjoy learning Data Science")
['Data Science']
re.sub(r"iceScream", "ice cream", "I really like ice-cream")
'I really like ice cream'

Let's practice!

Repetitions
Data Science

Repeated characters
Validate the following string:

Repeated characters
Validate the following string: import re
password = "password1234"
re.search(r"wwwwwwwwdddd", password)
<_sre.SRE_Match object; span=(0, 12), match='password1234'>

Repeated characters
Validate the following string: import re
password = "password1234"
re.search(r"w{8}d{4}", password)
<_sre.SRE_Match object; span=(0, 12), match='password1234'>
Quanti ers:
A metacharacter that tells the regex
engine how many times to match a
character immediately to its left.

Quanti ers
Once or more: +
text = "Date of start: 4-3. Date of registration: 10-04."
re.findall(r" ", text)

Quanti ers
Once or more: +
re.findall(r"d+- ", text)

Quanti ers
Once or more: +
re.findall(r"d+-d+", text)
['4-3', '10-04']

Quanti ers
Zero times or more: *
my_string = "The concert was amazing! @ameli!a @joh&&n @mary90"
re.findall(r"@w+W*w+", my_string)
['@ameli!a', '@joh&&n', '@mary90']

Quanti ers
Zero times or once: ?
text = "The color of this image is amazing. However, the colour blue could be brighter."
re.findall(r"colou?r", text)
['color', 'colour']

Quanti ers
n times at least, m times at most : {n, m}
phone_number = "John: 1-966-847-3131 Michelle: 54-908-42-42424"
re.findall(r" ", phone_number)

Quanti ers
re.findall(r"d{1,2}- ", phone_number)

Quanti ers
re.findall(r"d{1,2}-d{3}- ", phone_number)

Quanti ers
re.findall(r"d{1,2}-d{3}-d{2,3}-d{4,}", phone_number)
['1-966-847-3131', '54-908-42-42424']

Quanti ers
Immediately to the left
r"apple+" : + applies to e and not to apple

Regex
metacharacters
Data Scientist

Looking for patterns
Two different operations to nd a match:
re.search(r"d{4}", "4506 people attend the show
<_sre.SRE_Match object; span=(0, 4), match='4506
re.search(r"d+", "Yesterday, I saw 3 shows")
<_sre.SRE_Match object; span=(17, 18), match='3'>
re.match(r"d{4}", "4506 people attend the show"
<_sre.SRE_Match object; span=(0, 4), match='4506
print(re.match(r"d+","Yesterday, I saw 3 shows"
None

Special characters
Match any character (except newline): .
my_links = "Just check out this link: www.amazingpics.com. It has amazing photos!"
re.findall(r"www com", my_links)

Special characters
Match any character (except newline): .
my_links = "Just check out this link: www.amazingpics.com. It has amazing photos!"
re.findall(r"www.+com", my_links)
['www.amazingpics.com']

Special characters
Start of the string: ^
my_string = "the 80s music was much better that the 90s"
re.findall(r"thesd+s", my_string)
['the 80s', 'the 90s']
re.findall(r"^thesd+s", my_string)
['the 80s']

Special characters
End of the string: $
my_string = "the 80s music hits were much better that the 90s"
re.findall(r"thesd+s$", my_string)
['the 90s']

Special characters
Escape special characters:
my_string = "I love the music of Mr.Go. However, the sound was too loud."
print(re.split(r".s", my_string))
['', 'lov', 'th', 'musi', 'o', 'Mr.Go', 'However', 'th', 'soun', 'wa', 'to', 'loud.']
print(re.split(r".s", my_string))
['I love the music of Mr.Go', 'However, the sound was too loud.']

OR operator
Character: |
my_string = "Elephants are the world's largest land animal! I would love to see an elephant one day"
re.findall(r"Elephant|elephant", my_string)
['Elephant', 'elephant']

OR operator
Set of characters: [ ]
my_string = "Yesterday I spent my afternoon with my friends: MaryJohn2 Clary3"
re.findall(r"[a-zA-Z]+d", my_string)
['MaryJohn2', 'Clary3']

OR operator
my_string = "My&name&is#John Smith. I%live$in#London."
re.sub(r"[#$%&]", " ", my_string)
'My name is John Smith. I live in London.'

OR operand
^ transforms the expression to negative
my_links = "Bad website: www.99.com. Favorite site: www.hola.com"
re.findall(r"www[^0-9]+com", my_links)
['www.hola.com']

Greedy vs. non-
greedy matching
Data Scientist

Greedy vs. non-greedy matching
Two types of matching methods:
Greedy
Non-greedy or lazy
Standard quanti ers are greedy by default: * , + , ? , {num, num}

Greedy matching
Greedy: match as many characters as possible
Return the longest match
import re
re.match(r"d+", "12345bcada")

Greedy matching
Backtracks when too many character matched
Gives up characters one at a time
import re
re.match(r".*hello", "xhelloxxxxxx")
<_sre.SRE_Match object; span=(0, 6), match='xhello'>

Non-greedy matching
Lazy: match as few characters as needed
Returns the shortest match
Append ? to greedy quanti ers
import re
re.match(r"d+?", "12345bcada")

Non-greedy matching
Backtracks when too few characters matched
Expands characters one a time
import re
re.match(r".*?hello", "xhelloxxxxxx")
<_sre.SRE_Match object; span=(0, 6), match='xhello'>

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