肯定的な先読み: `(?=...)`

A positive lookahead (?=...) is a zero-width assertion: it checks that a certain pattern follows the current position, but does not consume those characters.

Pattern: \d+(?= euro)
Sample:  Prezzo 100 euro, sconto 25 euro, totale 75 dollari.
                ^^^                 ^^

The lookahead (?= euro) requires that the digits be followed by euro, but the match includes only the digits. 75 dollari does not match (no euro).

Why "zero-width"

Think of the lookahead as a condition on the position, not as a part of the match:

• The match stops before the lookahead.
• The position "after the match" is the start of the text inspected by the lookahead.
• The next match attempt with the g flag will resume from there.

This makes it perfect for extracting values without their context: prices without the currency, words before a punctuation mark, and so on.

Lookahead features and zero-width scan

The positive lookahead (?=...) guarantees that the specified pattern follows the current point, but scanning resumes from the position before the lookahead. This prevents "consuming" parts of the text that might be needed for subsequent matches.

Try it

Review exercise

Additional challenge