Regular Expressions

Regular expressions are a mini language for pattern matching. They are used in searching for patterns of text, replacing a pattern with another etc.

In Python, the built-in module re provides support for regular expressions.

Introduction

Suppose we want to find all the numbers in a sentence.

sentence = "10 apples and 20 mangoes"

It would be too cumbersome to do that with usual programming techniques.

Let’s see how we can do that using regular expressions.

import re

re.findall("\d+", sentence)

In the above example, the pattern \d matches any digit and \d+ matches one or more digits. We’ll learn more about the syntax of regular expressions in a while.

Suppose we want to mask all the numbers in a sentence.

re.sub("\d", "X", sentence)

Syntax

Regular expressions contain both simple and special characters. Simple characters like

1. Simple characters match themselves

The Python API

The re module provides the following functions to work with regular expressions.

findall

Finds all the occurances of a pattern in a string.

re.findall("\d+", "10 apples and 20 mangoes")
['10', '20']

match

Match a regular expression at the beginning of a string.

re.match("\d+", )

search

sub

split

compile

Advanced Usage

Example: Parse git log

Example: Extract Links

Problems

Problem: Write a function squeeze to replace multiple continuous whitespace with a single space.

>>> squeeze("a  b    c d")
'a b c d'

Problem: Write a function make_slug that takes title as argument and converts that into a nice name that can be used as part of URL. Make sure the the slug is in lower case.

>>> make_slug("Advanced python")
'advanced-python'
>>> make_slug("Hello, World!")
'hello-world'
>>> make_slug("1 + 2 = 3 !")
'1-2-3'
>>> make_slug("https://google.com/")
'https-google-com'