Regular Expressions in Python

Complete guide to Python Regular Expressions using the re module: syntax, patterns, flags, matching, searching, replacing, groups, quantifiers, anchors, lookarounds, validation, and real-world use cases.

Introduction to Regular Expressions

A regular expression (regex) is a pattern used for matching and manipulating text.

1. Basic Matching with re

Use re.search() for matching.

Creating Regex Patterns

Patterns are plain strings passed to re functions.

1. Raw Strings (r"")

Avoid escape issues.

Core re Functions

Python provides multiple regex functions.

1. search() vs match()

Difference in behavior.

2. findall()

Return all matches.

3. sub()

Replace matches.

4. finditer()

Iterator of match objects.

Regex Flags

Modify pattern behavior using flags.

1. Common Flags

IGNORECASE, MULTILINE, DOTALL.

Character Classes

Match specific character sets.

1. Common Classes

\d, \w, \s

Quantifiers

Control repetition.

1. Common Quantifiers

* + ? {n,m}

Anchors

Match positions in string.

1. ^ and $

Start and end anchors.

Groups & Named Groups

Capture sub-patterns.

1. Capturing Groups

Access with group().

2. Named Groups

Use (?P<name>).

Lookahead & Lookbehind

Match based on surrounding context.

1. Positive Lookahead

Use (?=...).

2. Positive Lookbehind

Use (?<=...).

Validation Examples

Common real-world patterns.

1. Basic Email Validation

Simple pattern.

2. Numeric String Validation

Digits only.