JS REGEX 08: Mastering Special RegEx Tricks: Captures, Groups and Peeks
When you’re diving into regular expressions (RegEx), there’s a moment when simple pattern matching just isn’t enough. You want more control — to grab just the piece you care about, group things smartly, or match based on what comes next without including it. That’s where RegEx groups and look-aheads become your secret weapons.
In this lesson, we’ll break down three powerful tricks:
🔹 1. Capturing Groups ( ): “Highlight This Bit for Me!”
Use capturing groups when you want to remember (or reuse) a specific part of your match.
🧠 How it works:
Parentheses () are used to capture part of your pattern. That captured part can be reused in code (match[1], match[2], etc.) or in a replacement string like $1.
🔍 Example:
(John) (Doe)
This matches the full string “John Doe”, but:
match[0]is"John Doe"(the entire match)match[1]is"John"match[2]is"Doe"
💡 You could use this to switch the names: "Doe, John" using $2, $1 in a replace.
🔹 2. Non-Capturing Groups (?: ): “Group This, But Don’t Save It!”
Sometimes you want to group part of your regex for logic, not for capture.
🧠 How it works:
Add ?: inside your parentheses: (?: ). This groups the content but doesn’t store it in your match results.
🔍 Example:
(?:red|blue) car
This matches “red car” or “blue car”, but you don’t capture “red” or “blue” as a separate group.
Why? Maybe you only care about capturing the word “car” later in a bigger expression. Keeping your capture groups clean makes your code easier to manage.
🔹 3. Look-Aheads (?= ) and (?! ): “Just Peek Ahead”
Look-aheads let you say: “Match this only if the next part is (or isn’t) something.”
They check the next part of the string without including it in the match.
✅ Positive Look-Ahead (?= ):
Match a pattern only if it’s followed by something specific.
apple(?= pie)
✔️ Matches "apple" in "apple pie"
✖️ Won’t match "apple juice"
❌ Negative Look-Ahead (?! ):
Match a pattern only if it’s not followed by something.
cat(?! dog)
✔️ Matches "cat" in "cat mouse"
✖️ Doesn’t match "cat" in "cat dog"
💡 Recap
| Trick | Symbol | Use Case |
|---|---|---|
| Capturing Group | ( ) |
Save and reuse parts of the match |
| Non-Capturing Group | (?: ) |
Group for logic without capturing |
| Positive Look-Ahead | (?= ) |
Match only if followed by something |
| Negative Look-Ahead | (?! ) |
Match only if not followed by something |
🧪 Practice Questions
Try writing RegEx patterns to solve the following challenges.
1. Capture First and Last Name
Match a full name like "Sarah James" and capture both parts separately.
2. Match “cat” only when not followed by “fish”
In a sentence like "cat dog" or "catfish", match only "cat" when it’s not part of "catfish".
3. Match “blue” or “green” shirts, but don’t capture the color
You’re scanning product names like "blue shirt" or "green shirt". Match the phrase, but capture only "shirt".
4. Rearrange a name from “Alice Smith” to “Smith, Alice” using replace
Use a RegEx pattern that captures both parts and rearranges them.
5. Match the word “cake” only if followed by “shop”
You want to find the word “cake” in “cake shop” but ignore it in “cake recipe”.
📬 Contact Me
📚Request a tutorial • 📩Send me a DM
