What is positive lookahead assertions in regular expressions .

 

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Positive lookahead in regular expressions is a zero-width assertion that checks whether a certain pattern follows the current position — without including it in the match.

It’s written like this:

(?=...)

Whatever goes inside ... must be true for the match to succeed — but it is not consumed as part of the match.

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🧠 Key idea

Think of it as saying:

“Match this… only if it is followed by that.”

But we don’t actually capture that in the final result.

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✅ Simple example

Pattern:

\d(?=px)

Text:

5px 10pt 8px

Matches:

5
8

Explanation:

• \d → match a digit

• (?=px) → only if it is immediately followed by the letters px

So it matches the 5 in 5px and the 8 in 8px, but not the 10 in 10pt, because that digit is not followed by px.

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💡 Another example — match words followed by !

Pattern:

\w+(?=!)

Text:

Wow! Amazing! Nice

Matches:

Wow
Amazing

The lookahead checks for ! but doesn’t include it in the match.

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🔍 Important properties

• Zero-width — it does not consume characters

• Does NOT capture the lookahead text

• Only checks what comes next

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🚫 Contrast: Negative lookahead

Positive lookahead:

(?=...)

means must be followed by ...

Negative lookahead:

(?!...)

means must NOT be followed by ...

Example:

foo(?!bar)

matches foo only when it is NOT followed by bar.

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📌 Common real-world uses

✔ Validate password rules
✔ Ensure a word is followed by another
✔ Match text before certain patterns
✔ Conditional matching without consuming text

Example — password must include a digit:

^(?=.*\d).+$ 

Meaning:

• (?=.*\d) → there must be at least one digit

• the rest just matches the password

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📝 Summary

Positive lookahead:

Feature	Description
Syntax	(?=...)
Purpose	Ensure pattern follows
Consumes text?	❌ No
Captures text?	❌ No
Works like	A condition

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