Lookahead

A lookahead checks what comes immediately after the current position. It is written inside angle brackets:

<?before …> — succeeds if the text ahead matches (positive lookahead)
<!before …> — succeeds if the text ahead does not match (negative lookahead)

The assertion itself matches no characters; it only tests the position. For example, match a number only when it is followed by the word dollars:

say '100 dollars' ~~ / \d+ <?before ' dollars'> /; # ｢100｣

The match is just 100 — the text dollars was checked but not included.

Because the lookahead only asserts what follows, the same number matches or not depending on the currency after it:

say so '100 euro'    ~~ / \d+ <?before ' euro'> /; # True
say so '100 dollars' ~~ / \d+ <?before ' euro'> /; # False

The digits are identical in both strings, but the assertion accepts 100 only when euro comes next.

The negative form is useful for “… but not when followed by …”. Match cat only when it is not followed by a digit:

say so 'cats' ~~ / cat <!before \d> /; # True
say so 'cat5' ~~ / cat <!before \d> /; # False

In cat5 the lookahead sees a digit right after cat, so the negative assertion fails and the whole match fails.

Practice

Complete the quiz that covers the contents of this topic.