Lookbehind: `(?<=...)` `(?<!...)`

Lookbehinds are the mirror image of lookaheads: they look before the current position, still zero-width.

• (?<=...) -- positive lookbehind: the pattern must precede.
• (?<!...) -- negative lookbehind: the pattern must NOT precede.

Pattern: (?<=\$)\d+
Sample:  Costo $100, sconto -25, totale $75.
                ^^^                  ^^

The $ is not in the match, but works as a condition: only digits preceded by the dollar sign match.

Negative lookbehind

Pattern: (?<!\w)\d+
Sample:  Codice abc123 e numero 456.
                              ^^^

(?<!\w) requires that before the digits there be NO word character: abc123 is excluded (before 123 there is c, a \w), 456 matches (before there is a space).

Key difference vs non-capturing group

\\$\\d+         matches "$100": the match includes the dollar sign.
(?<=\\$)\\d+    matches "100":  the match contains ONLY the digits.

If you need to extract the "clean" value without the prefix, lookbehind is the right choice.

Lookbehind compatibility in JavaScript

Positive (?<=...) and negative (?<!...) lookbehind check what precedes the current position. In JavaScript, they are supported starting from ES2018; in older browsers or legacy Node.js engines, they would cause syntax compilation errors.

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Review exercise

Additional challenge