2. Introduction to Regular Expressions
What is a Regular Expression?
• A language to specify the rules for the set of possible strings that you want to search in a corpus or text
• A regular expression (regex) is a sequence of characters that form a search pattern.
• Used for string searching and manipulation in programming languages.
3. Regex Module in Python
• Functions available: match(), search(), findall(), compile(), etc.
Common Functions in Regex
Syntax: re.match(pattern, string)
Matches a pattern only at the beginning of the string.
import re
re.match(r"d+", "123abc") # Matches '123'
• Python’s re module is used to work with regex patterns.
re.match()
4. re.search()
Syntax: re.search(pattern, string)
Searches for the first location where the pattern matches.
import re
re.search(r"d+", "Order number: 56789")
It returns a match object for 56789
re.findall()
Syntax: re.findall(pattern, string)
Returns all matches of a pattern in the string as a list.
re.findall(r"d+", "abc123def456") # Returns ['123', '456']
5. re.finditer()
Syntax: re.finditer(pattern, string)
Returns an iterator yielding match objects for all matches.
import re
for match in re.finditer(r"[A-Za-z]+", "123abc456def789ghi"):
print(match.group())
# Outputs 'abc', 'def', and 'ghi'
re.compile()
Syntax: re.compile(pattern)
Compiles a regex pattern for repeated use.
import re
pattern = re.compile(r"Tw+")
text = "Today is Tuesday, and tomorrow is Thursday."
matches = pattern.findall(text)
print(matches) # Outputs: ['Today', 'Tuesday', 'Thursday']
6. re.sub
Purpose: Replaces occurrences of a pattern in a string with a specified replacement.
import re
text = "I have a cat and a cat."
result = re.sub(r"cat", "dog", text) # Replaces 'cat' with 'dog'
print(result)
Output: "I have a dog and a dog."
re.split
Purpose: Splits a string into a list using a specified pattern as the delimiter.
import re
text = "apple,orange;banana|grape"
result = re.split(r"[;,|]", text) # Splits on commas, semicolons, or pipes
print(result)
Output: ['apple', 'orange', 'banana', 'grape']
7. • re.IGNORECASE (or re.I): Ignores case during matching.
• re.MULTILINE (or re.M): Treats the string as multiple lines.
• re.DOTALL (or re.S): Allows . to match newline characters
Flags in Regex
Common Flags
![re.search()
Syntax: re.search(pattern, string)
Searches for the first location where the pattern matches.
import re
re.search(r"d+", "Order number: 56789")
It returns a match object for 56789
re.findall()
Syntax: re.findall(pattern, string)
Returns all matches of a pattern in the string as a list.
re.findall(r"d+", "abc123def456") # Returns ['123', '456']](https://image.slidesharecdn.com/commonregexppt-250708092508-dccbaf3f/85/Common-regex-pp-functions-wweweewwt-pptx-4-320.jpg)
![re.finditer()
Syntax: re.finditer(pattern, string)
Returns an iterator yielding match objects for all matches.
import re
for match in re.finditer(r"[A-Za-z]+", "123abc456def789ghi"):
print(match.group())
# Outputs 'abc', 'def', and 'ghi'
re.compile()
Syntax: re.compile(pattern)
Compiles a regex pattern for repeated use.
import re
pattern = re.compile(r"Tw+")
text = "Today is Tuesday, and tomorrow is Thursday."
matches = pattern.findall(text)
print(matches) # Outputs: ['Today', 'Tuesday', 'Thursday']](https://image.slidesharecdn.com/commonregexppt-250708092508-dccbaf3f/85/Common-regex-pp-functions-wweweewwt-pptx-5-320.jpg)
![re.sub
Purpose: Replaces occurrences of a pattern in a string with a specified replacement.
import re
text = "I have a cat and a cat."
result = re.sub(r"cat", "dog", text) # Replaces 'cat' with 'dog'
print(result)
Output: "I have a dog and a dog."
re.split
Purpose: Splits a string into a list using a specified pattern as the delimiter.
import re
text = "apple,orange;banana|grape"
result = re.split(r"[;,|]", text) # Splits on commas, semicolons, or pipes
print(result)
Output: ['apple', 'orange', 'banana', 'grape']](https://image.slidesharecdn.com/commonregexppt-250708092508-dccbaf3f/85/Common-regex-pp-functions-wweweewwt-pptx-6-320.jpg)
