Week 7: Regular Expressions

Regular Expressions

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Know as a regex is just a pattern that we can use to match strings. It is a powerful tool that allows us to search for strings that match a certain pattern.

Validation with Regular Expressions

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email = input("Whats your email? ").strip()

username, domain = email.split("@")

if username and domain.ends with(".edu"):
    print("Valid")
else:
    print("Invalid")

re Library

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This library let us define a pattern that we can use to match strings.

Documentation

• re.search(pattern, string, flags=0)

import re

email = input("Whats your email? ").strip()

if re.search("@", email):
    print("Valid")
else:
    print("Invalid")

Regular Expressions Patterns

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Symbols

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Code using Regular Expression Patterns

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import re

email = input("Whats your email? ").strip()

if re.search(r".+@.+\.edu", email):
    print("Valid")
else:
    print("Invalid")

Matching Start and End

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• ^ Matches the start of the string.
• $ Matches the end of the string.

import re

email = input("Whats your email? ").strip()

if re.search(r"^.+@.+\.edu$", email):
    print("Valid")
else:
    print("Invalid")

Sets of Characters

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• [] Matches any one of the characters inside the brackets.
• [^] Matches any character not inside the brackets.

1. [^@]+

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import re

email = input("Whats your email? ").strip()

if re.search(r"^[^@]+@[^@]+\.edu$", email):
    print("Valid")
else:
    print("Invalid")

2. [a-zA-Z0-9_]+

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import re

email = input("Whats your email? ").strip()

if re.search(r"^[a-zA-Z0-9_]+@[a-zA-Z0-9_]+\.edu$", email):
    print("Valid")
else:
    print("Invalid")

Character Classes

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• \d Matches any digit; equivalent to [0-9].
• \D Matches any non-digit; equivalent to [^0-9].
• /s Matches any whitespace character (spaces, tabs, newlines).
• \S Matches any non-whitespace character.
• \w Matches any word character (alphanumeric plus underscore); equivalent to [a-zA-Z0-9_].
• \W Matches any non-word character; equivalent to [^a-zA-Z0-9_].

import re

email = input("Whats your email? ").strip()

if re.search(r"^\w+@\w+\.edu$", email):
    print("Valid")
else:
    print("Invalid")

Flags

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• re.IGNORECASE Makes the pattern case-insensitive.
• re.MULTILINE Makes the pattern match the start and end of each line.
• re.DOTALL Makes the . character match any character, including newlines.

import re

email = input("Whats your email? ").strip()

if re.search(r"^\w+@\w+\.edu$", email, re.IGNORECASE):
    print("Valid")
else:
    print("Invalid")

Groups

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• A|B Matches either the pattern before or the pattern after the pipe.
• () Groups patterns and captures the matched text.
• (?:...) Groups patterns without capturing the matched text.

import re

email = input("Whats your email? ").strip()

if re.search(r"^(\w|\.)+@(\w+\.)?\w+\.edu$", email, re.IGNORECASE):
    print("Valid")
else:
    print("Invalid")

Email Address Validation

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This is the regular expression pattern that the browsers uses to validate email addresses.

^[a-zA-Z0-9.!#$%&'*+\/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$

match, fullmatch

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• re.match(pattern, string, flags=0) Matches the pattern at the start of the string.
• re.fullmatch(pattern, string, flags=0) Matches the pattern against the whole string.

Capturing Groups

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• When we use () we are creating a capturing group that allows us to extract the matched text.
• := is the walrus operator that allows us to assign a value to a variable and use it in the same line.

import re

name = input("Whats your name? ").strip()
if matches := re.search(r"^(.+), *(.+)$", name):
    name = matches.group(2) + " " + matches.group(1)

print(f"Hello, {name}")

Extracting from Strings

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1. re.sub

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• .re.sub(pattern, repl, string, count=0, flags=0) Replaces the matched text with the replacement text.

import re

url = input("URL: ").strip()

username = re.sub(r"^(https?://)?(www\.)?twitter\.com/", "", url)
print(f"Username: {username}")

2. re.search

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• re.search(pattern, string, flags=0) Searches for the pattern in the string.

import re

url = input("URL: ").strip()

if matches := re.search(r"^https?://?(?:www\.)?twitter\.com/([a-z0-9_]+)", url, re.IGNORECASE)
    print(f"Username: {matches.group(1)}")

Conclusion

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There are other other functions

• re.split(pattern, string, maxsplit=0, flags=0) Splits the string at the matches of the pattern.
• re.findall(pattern, string, flags=0) Finds all the matches of the pattern in the string.