What are Python Regular Expressions (Regex)?

Why Is Python Regex Tester Important for Test Automation?

When you’re doing test automation, regex is essential because it allows you to automate text and data validation. Instead of doing that by hand on each piece of input data, you can use regex to check if the input matches a pattern (to some extent), extract useful information from it, and even manipulate and manage strings before testing.

The following is why you should include Python regex checker in your automation testing toolkit:

How to Use Python Regex in Test Automation?

Let’s take a practical approach to using Python regex in your test automation scripts. The key is knowing how to define a regex pattern, select the right re function, and integrate it into your automated tests.

Mastering regex components in the vast landscape of Python programming provides a powerful arsenal for automation testers. Let’s delve into these components with practical examples, showcasing how they can be combined to perform versatile string manipulations and pattern matching.

Step 1: Import the re Module

The first step is importing Python’s re module. It’s the built-in module that lets you work with regex.

import re

Step 2: Define the Regex Pattern

A regex pattern is a string that defines the search behavior you want. You’ll need to create a pattern that matches the data you’re looking for. For example, here’s a regex pattern that matches a Social Security Number (SSN) in the format XXX-XX-XXXX:

pattern = r'\d{3}-\d{2}-\d{4}'

This pattern tells Python to look for three digits, followed by a hyphen, two digits, another hyphen, and four digits.

Step 3: Choose the Right Regex Function

Once you have your pattern, you’ll use one of the re functions to perform the search. The most used functions are:

For example, let’s use re.search() to find a SSN in a string:

text = "My SSN is 123-45-6789."
match = re.search(r'\d{3}-\d{2}-\d{4}', text)
if match:
    print(f'Match found: {match.group()}')

This will search for a pattern in the string and return the first match.

Common Python Regex Examples

Now that you know the basics, let’s look at some practical Python regex examples to see how regex can be applied in test automation.

1. Python Regex Email Validation

Validating email addresses is a frequent task in automation testing. Here’s how you can use regex to check if an email is formatted correctly:

import re
 
email_pattern = r'^[\w\.-]+@[\w\.-]+\.\w+$'
email = "test@example.com"
 
if re.match(email_pattern, email):
    print("Valid email address")
else:
    print("Invalid email address")

This regex checks that the email has the correct format, such as test@example.com.

2. Extracting Hyperlinks

Sometimes, you may need to extract URLs from text. This can be done easily with regex:

text = "Visit our website at https://www.accelq.com for more details."
url_pattern = r'https?://\S+'
 
urls = re.findall(url_pattern, text)
print(urls)  # Output: ['https://www.accelq.com']

This pattern matches any string starting with http or https and followed by non-whitespace characters.

3. Validating and Extracting SSNs

If you need to validate and extract SSNs in a string, here’s an example:

import re
 
text = "SSNs: 123-45-6789 and 987-65-4321"
ssn_pattern = r'\d{3}-\d{2}-\d{4}'
 
ssn_list = re.findall(ssn_pattern, text)
print(ssn_list)  # Output: ['123-45-6789', '987-65-4321']

This regex will match both SSN formats, whether they include hyphens or not.

Advanced Python Regex Features: Lookahead and Lookbehind

Lookahead/Lookbehind in Python Regex

Lookahead and lookbehind are advanced regex features that let you check if a string is followed or preceded by another pattern, without including it in the match.

Lookahead Example

In lookahead, we match a pattern only if it’s followed by another pattern. Here’s how it works:

pattern = r'word(?=\s)'  # Matches 'word' only if followed by a space
text = 'word followed by space'
match = re.search(pattern, text)

Lookbehind Example

Lookbehind checks for a pattern that precedes the match:

pattern = r'(?<=\s)word'  # Matches 'word' only if it's preceded by a space
text = 'space before word'
match = re.search(pattern, text)

Tools to Test Your Regex Patterns

Testing regular expressions can be a bit of a pain. The good news is that some tools make this less painful:

These are the tools that let you easily test your regex before putting them into Python scripts.