Here’s a clear, easy-to-use guide to positive lookahead assertions in regular expressions:
✅ What is a Positive Lookahead?
A positive lookahead checks whether a certain pattern comes after the current position — without including it in the match.
The syntax is:
(?=pattern)
Think of it as saying:
“Match this… only if it is followed by that.”
Lookaheads are zero-width assertions — they do not consume characters. They only peek ahead.
🔍 Simple Example
Match the word cat only when followed by s
Regex:
cat(?=s)
Text:
cats cat catapult scatter
Matches:
cat (in cats)
Not these:
cat (alone)
cat (in catapult)
Because only the first cat is directly followed by s.
🛠 Practical Uses
1️⃣ Ensure a password contains a number
Regex:
^(?=.*[0-9]).+$
Meaning:
(?=.*[0-9])→ somewhere ahead, there must be a digit.+→ match the whole string
2️⃣ Match emails from a specific domain
Match username only if followed by @gmail.com
.+(?=@gmail\.com)
Text:
alex@gmail.com
sam@yahoo.com
Matches only:
alex
3️⃣ Match a word only when followed by punctuation
word(?=[.!?])
Matches word in:
word! word? word.
But not in:
word is here
🔄 Compare: Positive vs Negative Lookahead
| Type | Syntax | Meaning |
|---|---|---|
| Positive Lookahead | (?=...) | Must be followed by… |
| Negative Lookahead | (?!...) | Must not be followed by… |
Example negative:
cat(?!s)
Means “cat not followed by s”.
💡 Key Things to Remember
Lookaheads do not consume characters
They work at the same position
They can be combined
Supported in most engines (JS, Python, PCRE, etc.)
🧪 Example in JavaScript
const regex = /cat(?=s)/g;
console.log("cats cat".match(regex));
// Output: ["cat"]
🧪 Example in Python
import re
print(re.findall(r"cat(?=s)", "cats cat"))
# Output: ['cat']
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