Python Regular Expressions

Are you tired of manually searching through strings to find specific patterns? Do you want to automate the process of finding and manipulating text in your Python code? If so, then you need to learn about Python regular expressions!

Python regular expressions are a powerful tool for searching and manipulating text in Python. They allow you to search for specific patterns in strings, replace text with other text, and extract information from strings. In this article, we'll explore the basics of Python regular expressions and show you how to use them in your own code.

What are Regular Expressions?

Regular expressions, also known as regex or regexp, are a sequence of characters that define a search pattern. They are used to search for specific patterns in text, such as email addresses, phone numbers, or URLs. Regular expressions are supported by many programming languages, including Python.

A regular expression is made up of two types of characters: literals and metacharacters. Literals are characters that match themselves, such as "a", "b", or "c". Metacharacters are special characters that have a specific meaning in regular expressions, such as "^", "$", or ".".

Basic Regular Expression Syntax

Let's start with some basic regular expression syntax. The simplest regular expression is a literal, which matches itself. For example, the regular expression "hello" matches the string "hello".

import re

pattern = "hello"
string = "hello world"

match = re.search(pattern, string)

if match:
    print("Match found!")
else:
    print("Match not found.")

In this example, we import the re module, which provides support for regular expressions in Python. We define a pattern that matches the string "hello", and a string that contains the word "hello". We then use the re.search() function to search for the pattern in the string. If a match is found, we print "Match found!". Otherwise, we print "Match not found.".

Metacharacters

Now let's look at some metacharacters. Metacharacters have a special meaning in regular expressions, and are used to match specific patterns in text.

The Dot Metacharacter

The dot metacharacter (".") matches any single character, except for a newline character. For example, the regular expression "h.llo" matches "hello", "hallo", and "hxllo", but not "h\nllo".

import re

pattern = "h.llo"
string1 = "hello world"
string2 = "hallo there"
string3 = "hxllo"

match1 = re.search(pattern, string1)
match2 = re.search(pattern, string2)
match3 = re.search(pattern, string3)

if match1:
    print("Match found in string1!")
else:
    print("Match not found in string1.")

if match2:
    print("Match found in string2!")
else:
    print("Match not found in string2.")

if match3:
    print("Match found in string3!")
else:
    print("Match not found in string3.")

In this example, we define a pattern that matches the string "h.llo". We then define three strings that contain variations of "hello". We use the re.search() function to search for the pattern in each string, and print a message if a match is found.

The Caret Metacharacter

The caret metacharacter ("^") matches the beginning of a string. For example, the regular expression "^hello" matches the string "hello world", but not "world hello".

import re

pattern = "^hello"
string1 = "hello world"
string2 = "world hello"

match1 = re.search(pattern, string1)
match2 = re.search(pattern, string2)

if match1:
    print("Match found in string1!")
else:
    print("Match not found in string1.")

if match2:
    print("Match found in string2!")
else:
    print("Match not found in string2.")

In this example, we define a pattern that matches the string "^hello". We then define two strings that contain variations of "hello". We use the re.search() function to search for the pattern in each string, and print a message if a match is found.

The Dollar Sign Metacharacter

The dollar sign metacharacter ("$") matches the end of a string. For example, the regular expression "world$" matches the string "hello world", but not "world hello".

import re

pattern = "world$"
string1 = "hello world"
string2 = "world hello"

match1 = re.search(pattern, string1)
match2 = re.search(pattern, string2)

if match1:
    print("Match found in string1!")
else:
    print("Match not found in string1.")

if match2:
    print("Match found in string2!")
else:
    print("Match not found in string2.")

In this example, we define a pattern that matches the string "world$". We then define two strings that contain variations of "world". We use the re.search() function to search for the pattern in each string, and print a message if a match is found.

The Asterisk Metacharacter

The asterisk metacharacter ("") matches zero or more occurrences of the preceding character. For example, the regular expression "ab" matches "a", "ab", "abb", "abbb", and so on.

import re

pattern = "ab*"
string1 = "a"
string2 = "ab"
string3 = "abb"
string4 = "abbb"

match1 = re.search(pattern, string1)
match2 = re.search(pattern, string2)
match3 = re.search(pattern, string3)
match4 = re.search(pattern, string4)

if match1:
    print("Match found in string1!")
else:
    print("Match not found in string1.")

if match2:
    print("Match found in string2!")
else:
    print("Match not found in string2.")

if match3:
    print("Match found in string3!")
else:
    print("Match not found in string3.")

if match4:
    print("Match found in string4!")
else:
    print("Match not found in string4.")

In this example, we define a pattern that matches the string "ab*". We then define four strings that contain variations of "ab". We use the re.search() function to search for the pattern in each string, and print a message if a match is found.

The Plus Sign Metacharacter

The plus sign metacharacter ("+") matches one or more occurrences of the preceding character. For example, the regular expression "ab+" matches "ab", "abb", "abbb", and so on, but not "a".

import re

pattern = "ab+"
string1 = "a"
string2 = "ab"
string3 = "abb"
string4 = "abbb"

match1 = re.search(pattern, string1)
match2 = re.search(pattern, string2)
match3 = re.search(pattern, string3)
match4 = re.search(pattern, string4)

if match1:
    print("Match found in string1!")
else:
    print("Match not found in string1.")

if match2:
    print("Match found in string2!")
else:
    print("Match not found in string2.")

if match3:
    print("Match found in string3!")
else:
    print("Match not found in string3.")

if match4:
    print("Match found in string4!")
else:
    print("Match not found in string4.")

In this example, we define a pattern that matches the string "ab+". We then define four strings that contain variations of "ab". We use the re.search() function to search for the pattern in each string, and print a message if a match is found.

The Question Mark Metacharacter

The question mark metacharacter ("?") matches zero or one occurrence of the preceding character. For example, the regular expression "ab?" matches "a" and "ab", but not "abb" or "abbb".

import re

pattern = "ab?"
string1 = "a"
string2 = "ab"
string3 = "abb"
string4 = "abbb"

match1 = re.search(pattern, string1)
match2 = re.search(pattern, string2)
match3 = re.search(pattern, string3)
match4 = re.search(pattern, string4)

if match1:
    print("Match found in string1!")
else:
    print("Match not found in string1.")

if match2:
    print("Match found in string2!")
else:
    print("Match not found in string2.")

if match3:
    print("Match found in string3!")
else:
    print("Match not found in string3.")

if match4:
    print("Match found in string4!")
else:
    print("Match not found in string4.")

In this example, we define a pattern that matches the string "ab?". We then define four strings that contain variations of "ab". We use the re.search() function to search for the pattern in each string, and print a message if a match is found.

Character Classes

Character classes are a way to match a set of characters. They are enclosed in square brackets ("[]"), and can contain a list of characters or ranges of characters.

Matching a Range of Characters

To match a range of characters, you can use a hyphen ("-"). For example, the regular expression "[a-z]" matches any lowercase letter.

import re

pattern = "[a-z]"
string1 = "hello world"
string2 = "HELLO WORLD"
string3 = "12345"

match1 = re.search(pattern, string1)
match2 = re.search(pattern, string2)
match3 = re.search(pattern, string3)

if match1:
    print("Match found in string1!")
else:
    print("Match not found in string1.")

if match2:
    print("Match found in string2!")
else:
    print("Match not found in string2.")

if match3:
    print("Match found in string3!")
else:
    print("Match not found in string3.")

In this example, we define a pattern that matches any lowercase letter. We then define three strings that contain variations of letters, uppercase letters, and numbers. We use the re.search() function to search for the pattern in each string, and print a message if a match is found.

Matching a Set of Characters

To match a set of characters, you can list them inside the square brackets. For example, the regular expression "[aeiou]" matches any vowel.

import re

pattern = "[aeiou]"
string1 = "hello world"
string2 = "HELLO WORLD"
string3 = "12345"

match1 = re.search(pattern, string1)
match2 = re.search(pattern, string2)
match3 = re.search(pattern, string3)

if match1:
    print("Match found in string1!")
else:
    print("Match not found in string1.")

if match2:
    print("Match found in string2!")
else:
    print("Match not found in string2.")

if match3:
    print("Match found in string3!")
else:
    print("Match not found in string3.")

In this example, we define a pattern that matches any vowel. We then define three strings that contain variations of letters, uppercase letters, and numbers. We use the re.search() function to search for the pattern in each string, and print a message if a match is found.

Negating a Set of Characters

To negate a set of characters, you can use the caret metacharacter ("^") at the beginning of the square brackets. For example, the regular expression "[^aeiou]" matches any character that is not a vowel.

import re

pattern = "[^aeiou]"
string1 = "hello world"
string2 = "HELLO WORLD"
string3 = "12345"

match1 = re.search(pattern, string1)
match2 = re.search(pattern, string2)
match3 = re.search(pattern, string3)

if match1:
    print("Match found in string1!")
else:
    print("Match not found in string1.")

if match2:
    print("Match found in string2!")
else:
    print("Match not found in string2.")

if match3:
    print("Match found in string3!")
else:
    print("Match not found in string3.")

In this example, we define a pattern that matches any character that is not a vowel. We then define three strings that contain variations of letters, uppercase letters, and numbers. We use the re.search() function to search for the pattern in each string, and print a message if a match is found.

Quantifiers

Quantifiers are used to specify how many times a character or group of characters should be matched.

Matching a Specific Number of Occurrences

To match a specific number of occurrences, you can use curly braces ("{}"). For example, the regular expression "a{3}" matches "aaa", but not "aa" or "aaaa".

import re

pattern = "a{3}"
string1 = "aaa"
string2 = "aa"
string3 = "aaaa"

match1 = re.search(pattern, string1)
match2 = re.search(pattern, string2)
match3 = re.search(pattern, string3)

if match1:
    print("Match found in string1!")
else:
    print("Match not found in string1.")

if match2:
    print("Match found in string2!")
else:
    print("Match not found in string2.")

if match3:
    print("Match found in string3!")
else:
    print("Match not found in string3.")

In this example, we define a pattern that matches the string "a{3}". We then define three strings that contain variations of "a". We use the re.search() function to search for the pattern in each string, and print a message if a match is found.

Matching a Range of Occurrences

To match a range of occurrences, you can use curly braces with two numbers separated by a comma ("{m,n}"). For example, the regular expression "a{2,4}" matches "aa", "aaa", and "aaaa", but not "a" or "aaaaa".

import re

pattern = "a{2,4}"
string1 = "aa"
string2 = "aaa"
string3 = "aaaa"
string4 = "a"
string5 = "aaaaa"

match1 = re.search(pattern, string1)
match2 = re.search(pattern, string2)
match3 = re.search(pattern, string3)
match4 = re.search(pattern, string4)
match5 = re.search(pattern, string5)

if match1:
    print("Match found in string1!")
else:
    print("Match not found in string1.")

if match2:
    print("Match found in string2!")
else:
    print("Match not found in string2.")

if match3:
    print("Match found in string3!")
else:
    print("Match not found in string3.")

if match4:
    print("Match found in string4!")
else:
    print("Match not found in string4.")

if match5:
    print("Match found in string5!")
else:
    print("Match not found in string5.")

In this example, we define a pattern that matches the string "a{2,4}". We then define five strings that contain variations of "a". We use the re.search() function to search for the pattern in each string, and print a message if a match is found.

Matching Zero or More Occurrences

To match zero or more occurrences, you can use the asterisk metacharacter (""). For example, the regular expression "a" matches "a", "aa", "aaa", and so on.

import re

pattern = "a*"
string1 = "a"
string2 = "aa"
string3 = "aaa"

match1 = re.search(pattern, string1)
match2 = re.search(pattern, string2)
match3 = re.search(pattern, string3)

if match1:
    print("Match found in string1!")
else:
    print("Match not found in string1.")

if match2:
    print("Match found in string2!")
else:
    print("Match not found in string2.")

if match3:
    print("Match found in string3!")
else:
    print("Match not found in string3.")

In this example, we define a pattern that matches the string "a*". We then define three strings that contain variations of "a". We use the re.search() function to search for the pattern in each string, and print a message if a match is found.

Matching One or More Occurrences

To match one or more occurrences, you can use the plus sign metacharacter ("+"). For example, the regular expression "a+" matches "a", "aa", "aaa", and so on, but not an empty string.

import re

pattern = "a+"
string1 = "a"
string2 = "aa"
string3 = "aaa"
string4 = ""

match1 = re.search(pattern, string1)
match2 = re.search(pattern, string2)
match3 = re.search(pattern, string3)
match4 = re.search(pattern, string4)

if match1:
    print("Match found in string1!")
else:
    print("Match not found in string1.")

if match2:
    print("Match found in string2!")
else:
    print("Match not found in string2.")

if match3:
    print("Match found in string3!")
else:
    print("Match not found in string3.")

if match4:
    print("Match found in string4!")
else:
    print("Match not found in string4.")

In this example, we define a pattern that matches the string "a+". We then define four strings that contain variations of "a". We use the re.search() function to search for the pattern in each string, and print a message if a match is found.

Groups

Groups are a way to capture parts of a regular

Editor Recommended Sites

AI and Tech News
Best Online AI Courses
Classic Writing Analysis
Tears of the Kingdom Roleplay
Prompt Catalog: Catalog of prompts for specific use cases. For chatGPT, bard / palm, llama alpaca models
Macro stock analysis: Macroeconomic tracking of PMIs, Fed hikes, CPI / Core CPI, initial claims, loan officers survey
Zero Trust Security - Cloud Zero Trust Best Practice & Zero Trust implementation Guide: Cloud Zero Trust security online courses, tutorials, guides, best practice
ML Privacy:
Dev Tradeoffs: Trade offs between popular tech infrastructure choices