Check what follows without taking it

x(?=y) matches x only if y comes next, but y is not part of the match. (?!y) is the negative.

Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.

What it is for

The number before a unit, a word not followed by a suffix, password rules like 'must contain a digit'.

How to think about it

Is the context part of the match, or only a condition? When you want a condition on the context but a clean match, use lookahead. When you need the context in the match, do not.

Worked example

pattern: \d+(?= ms)

Digits, but only if ' ms' follows.

text: took 250 ms

Matches 250 only.

pattern: ^(?=.*\d).{8,}$

At least 8 characters and somewhere a digit: the password rule shape.

Your turn

A word followed by a colon, without the colon.

\w+(:)

The trap

Lookbehind (?<=...) exists too, but not every engine supports it. Lookahead is universal.

Practise lookahead on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.