2024-12-16 Web Development, Productivity, Programming

Master Regex: A Practical Series with an Interactive Tester

By O. Wolfson

Regular expressions often seem intimidating at first glance. To make regex more accessible and actionable, I plan to write practical blog series that will walk us through real-world use cases. From matching email addresses to validating passwords, each post will focus on a specific task, providing step-by-step guidance and allowing us to test our newfound skills interactively here or on a regex test sitelike Regexr.

In this series, I'll cover everything from beginner-friendly patterns to advanced text manipulations, helping us master regex one task at a time. Use the Regex Tester to test your regex against a test string. Matches are highlighted in yellow.

Example Regex: \b(apple|banana)\b

Example String:

text
I have an apple, a banana, and 34 oranges. Today, I bought 12 more apples and sent an email to fruitlover@example.com to share my joy. The date was 2024-12-16, but I also noted it down as 16/12/2024 for my journal. "Fresh fruits are amazing," I thought to myself. I called my friend at +1-800-555-0199 and shared the news, as well as a story about how jogging in the morning helps me stay healthy. She replied with a hashtag: #FruitForLife, and sent me a link to a great website: https://www.healthyliving.com.

Regex Tester

Beginner Level: Simple Patterns

Match Specific Words or Phrases
- Task: Match occurrences of "apple" or "banana" in a text.
```
regex
\b(apple|banana)\b
```
Find Digits in Text
- Task: Extract all numbers from "I have 12 apples and 34 oranges."
```
regex
\d+
```

Locate Emails

Task: Identify valid email addresses in a block of text.

regex
[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}

Intermediate Level: Grouping and Quantifiers

Match Dates

Task: Match dates in formats like YYYY-MM-DD or DD/MM/YYYY.

regex
\b\d{4}-\d{2}-\d{2}\b

regex
\b\d{2}/\d{2}/\d{4}\b

Extract Words Between Quotation Marks
- Task: Capture content within double quotes.
```
regex
"([^"]+)"
```

Phone Number Validation

Task: Match phone numbers in different formats (e.g., +1-800-555-0199, (123) 456-7890).

regex
\+?\d{1,3}?[-.\s]?\(?\d{1,4}?\)?[-.\s]?\d{1,4}[-.\s]?\d{1,9}

Find Words Ending in Specific Letters
- Task: Identify words ending with "ing".
```
regex
\b\w+ing\b
```

Advanced Level: Advanced Patterns

Validate URLs

Task: Match valid URLs in a text.

regex
https?:\/\/[^\s/$.?#].[^\s]*

Extract Hashtags
- Task: Find hashtags in a block of social media posts.
```
regex
#\w+
```

Match Nested HTML Tags

Task: Capture content inside <div> tags.

regex
<div>(.*?)<\/div>

Highlight Incorrect Capitalization
- Task: Identify sentences that do not start with an uppercase letter.
```
regex
(?<![.?!]\s)[a-z]
```

Real-World Applications

Extract CSV Columns
- Task: Match and extract values in a CSV file.
```
regex
(?:^|,)([^,]*)
```
Filter Log Entries
- Task: Identify error messages in a system log.
```
regex
ERROR:\s.*
```

Match IP Addresses

Task: Find valid IPv4 addresses.

regex
\b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b

Search and Replace
- Task: Replace all occurrences of "foo" with "bar".
```
regex
foo
```
  Replacement:
```
text
bar
```

Bonus Challenges

Password Validation

Task: Match strong passwords (e.g., at least 8 characters, one uppercase letter, one digit).

regex
(?=.*[A-Z])(?=.*\d)[A-Za-z\d]{8,}

Clean Up Whitespace

Task: Remove extra spaces from a text.

regex
\s{2,}

Replacement:

text
(a single space)

Extract Domain Names

Task: Capture the domain part of an email address.

regex
@([a-zA-Z0-9.-]+\.[a-zA-Z]{2,})

Find Palindromes

Task: Match palindromic words (e.g., "madam", "racecar").

regex
\b(\w)(\w)?\w?\2\1\b

Scrape Markdown Links
- Task: Extract links from Markdown syntax [text](url).
```
regex
\[(.*?)\]\((.*?)\)
```

Please like this post if you want to see this series continue!