Lookahead and lookbehind in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a lookahead in regular expressions?

A lookahead checks if a certain pattern exists ahead in the text without including it in the match. It looks forward but does not consume characters.

beginner

What is a lookbehind in regular expressions?

A lookbehind checks if a certain pattern exists behind the current position without including it in the match. It looks backward but does not consume characters.

intermediate

How do you write a positive lookahead in PHP regex?

Use (?=pattern). For example, /foo(?=bar)/ matches 'foo' only if it is followed by 'bar'.

intermediate

How do you write a negative lookbehind in PHP regex?

Use (?<!pattern). For example, /(?<!foo)bar/ matches 'bar' only if it is NOT preceded by 'foo'.

intermediate

Why use lookahead and lookbehind instead of normal matching?

They let you check context around a match without including that context in the matched text. This helps when you want to find a pattern only if it is near another pattern, but you don't want to capture the nearby pattern.

Which syntax represents a positive lookahead in PHP regex?

A (?<=pattern)

B (?=pattern)

C (?!pattern)

D (?<!pattern)

A positive lookahead uses (?=pattern) to check if 'pattern' follows without including it in the match.

What does (?foo)bar match?

A Matches 'bar' only if NOT preceded by 'foo'

B Matches 'foo' followed by 'bar'

C Matches 'bar' only if preceded by 'foo'

D Matches 'bar' only if followed by 'foo'

(?

Which lookaround checks text after the current position?

A Neither

B Lookbehind

C Both lookahead and lookbehind

D Lookahead

Lookahead checks the text ahead (after) the current position.

What is the main difference between lookahead and normal matching?

A Lookahead does not consume characters, normal matching does

B Lookahead matches only digits

C Lookahead matches only at the start of the string

D Lookahead consumes characters in the match

Lookahead checks for a pattern ahead but does not include it in the matched text, unlike normal matching.

Which of these is NOT a valid lookaround syntax in PHP regex?

A (?<!pattern)

B (?<=pattern)

C (?<>pattern)

D (?=pattern)

(?<>pattern) is not a valid lookaround syntax.

Explain how lookahead and lookbehind help in matching patterns with context in PHP regex.

Think about checking surroundings without including them in the match.

Write a PHP regex example using positive lookahead and explain what it matches.

Try matching a word only if followed by another word.