Regular Expressions in Python

1. Importing the `re` Module

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

import re

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.

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".

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

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!")

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())