Basics

Lookarounds often cause confusion to new regex learners. There are four lookarounds:

(?<= … ) and (?= … ),

(?<! … ) and (?! … )

Collectively, lookbehinds and lookaheads are known as lookarounds. Let’s see the following table of examples:

Lookaround	Name	What it Does
(?=learn)	Lookahead	Asserts that what immediately follows the current position in the string is learn
(?<=learn)	Lookbehind	Asserts that what immediately precedes the current position in the string is learn
(?!learn)	Negative Lookahead	Asserts that what immediately follows the current position in the string is not learn
(?<!learn)	Negative Lookbehind	Asserts that what immediately precedes the current position in the string is not learn