Regular Expressions in Python

Regular expressions, or regex, provide a way to search for patterns within strings. In Python, the re module is used for regex-based pattern matching, enabling advanced string manipulation.

1. Importing the `re` Module

The re module in Python offers functions for working with regular expressions. Import it with:

import re

Module Basics Quiz

Which module is used for regular expressions in Python?

regex
re
regexp

What does the 'r' prefix before a regex pattern mean?

It makes the pattern required
It creates a raw string (treats \ literally)
It makes the search case-sensitive

2. Basic Pattern Matching

Use re.search() to find the first match of a pattern within a string:

import re

pattern = r"hello"
text = "hello world"
match = re.search(pattern, text)
if match:
    print("Match found:", match.group())

The r before the pattern indicates a raw string, which treats backslashes as literal characters.

Pattern Matching Quiz

What does re.search() return if no match is found?

None
An empty string
False

How do you access the matched text from a match object?

match.text()
match.group()
match.string()

3. Using `re.findall()`

re.findall() returns all matches of a pattern in a list:

text = "cat bat rat mat"
matches = re.findall(r"\b\w+at\b", text)
print(matches)  # Output: ['cat', 'bat', 'rat', 'mat']

This example uses the word boundary \b to match words ending in "at".

findall() Quiz

What does re.findall() return if no matches are found?

An empty list
None
False

What does \b represent in regex?

A backspace character
A word boundary
The beginning of a string

4. Replacing Text with `re.sub()`

Use re.sub() to replace matches in a string:

text = "I like cats"
new_text = re.sub(r"cats", "dogs", text)
print(new_text)  # Output: I like dogs

sub() Quiz

How many replacements does re.sub() make by default?

Only the first match
All matches
None unless specified

How would you limit replacements to just the first match?

re.sub(pattern, repl, text, count=1)
re.sub_first(pattern, repl, text)
re.sub(pattern, repl, text, limit=1)

5. Pattern Modifiers

Modifiers control the behavior of regex. Commonly used flags include:

re.IGNORECASE or re.I: Case-insensitive matching.
re.MULTILINE or re.M: Multi-line matching for patterns like ^ and $.

text = "Hello world"
match = re.search(r"hello", text, re.IGNORECASE)
if match:
    print("Case-insensitive match found!")

Modifiers Quiz

Which flag makes matching case-insensitive?

re.IGNORECASE
re.CASELESS
re.NOCASE

What does re.MULTILINE affect?

Behavior of ^ and $ anchors
Matching across multiple strings
Pattern compilation speed

6. Common Regex Patterns

\d: Matches any digit (0-9).
\w: Matches any alphanumeric character (a-z, A-Z, 0-9, _).
\s: Matches any whitespace character (space, tab, newline).
^: Matches the beginning of a string.
$: Matches the end of a string.

Example:

text = "My phone number is 123-456-7890"
pattern = r"\d{3}-\d{3}-\d{4}"
match = re.search(pattern, text)
if match:
    print("Phone number found:", match.group())

Patterns Quiz

What does \d match in regex?

Any digit (0-9)
Any decimal number
Any character

What does {3} in \d{3} mean?

Exactly 3 repetitions
At least 3 repetitions
Between 3 and 5 repetitions

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