Lesson 9

More Assertions

Introduction

That last lesson was heavy! Don't worry, this one is a lot shorter.

It covers a really easy way of 'looking around' the input string to find your match.

Lookahead

A 'lookahead' lets us match something only when it is immediately followed by something else in the string.

Cricually, the lookahead does not capture the text that follows, it only uses it for the match.

A lookahead is written in brackets like a group, but it starts with a ?=, e.g. (?=hello).

This expression matches one-or-more word characters (\w+) that are immediately followed by the "ing" ((?=ing)):

Notice how the 'ing' text is not captured, it's only used to match the characters before it.

Negative lookahead

Starting the lookahead with ?! makes it a 'negative lookahead'. That means it will only match when NOT immediately followed by the lookahead value.

This expression looks for two-or-more numbers ([0-9]{2,}) that are NOT followed by the text "kg" ((?!kg)):

Notice that the "75" from "75kg" is not to matched.

Lookbehind

As you have probably guess, this is the opposite of a 'lookahead'.

The 'lookbehind' matches something immediately preceded by the condition.

You can recognise a lookbehind by the ?<= at the start (the arrow is pointing backwards, indicating a lookbehind).

This expression matches one-or-more numbers ([0-9]+), but only when they are immediately preceded by a $ sign ((?<=\$))

(note that we need to escape the $ sign with a \. The $ is 'reserved' in regular expressions, so it becomes \$)

The number "100" is not captured, because it doesn't have the dollar sign before it.

Negative lookbehind

Lookbehinds also have a 'negative' version, indicated by a ?<! at the start.

This expression looks for two-or-more numbers ([0-9]{2,}), but only when they are not preceded by the $ sign ((?<!\$))

Mini Game