Negative lookahead: `(?!...)`

The negative lookahead (?!...) is the dark sister of the positive lookahead: it requires that the given pattern NOT follow the current position. It is still zero-width: no character is consumed.

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

Only 75 matches, because it is the only sequence of digits NOT followed by euro. The digits 100 and 25 are followed by euro and are excluded.

Typical negative lookahead patterns

• ^(?!.*errore).*$ -- a line that does NOT contain the word "errore".
• \b(?!the\b)\w+\b -- a word that is not the.
• (?!\s) -- the current position is NOT a whitespace (useful as a "trim").

Pattern: ^(?!.*\.bak$).*$
Sample (multiline with flag m):
  foto.jpg
  backup.bak
  documento.txt
Match: foto.jpg, documento.txt (excludes backup.bak)

Advanced exclusions via negative lookahead

The negative lookahead (?!...) is essential for implementing exclusion logic (like verifying that a password does not contain dictionary words or that an identifier is not a reserved keyword like class or function).

Try it

Review exercise

Additional challenge