Lookahead and lookbehind in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of positive lookahead regex in PHP

What is the output of this PHP code snippet?

$text = "apple pie and apple tart";
$pattern = '/apple(?= pie)/';
preg_match_all($pattern, $text, $matches);
print_r($matches[0]);

PHP

$text = "apple pie and apple tart";
$pattern = '/apple(?= pie)/';
preg_match_all($pattern, $text, $matches);
print_r($matches[0]);

A Array ( [0] => apple [1] => apple )

B Array ( [0] => pie )

C Array ( [0] => apple )

D Array ( )

💡 Hint

Lookahead checks what comes after the word without including it.

The pattern 'apple(?= pie)' matches 'apple' only if it is followed by ' pie'. In the text, only the first 'apple' is followed by ' pie', so only one match is found.

❓ Predict Output

intermediate

Output of negative lookbehind regex in PHP

What will this PHP code output?

$text = "cat bat rat mat";
$pattern = '/(?preg_match_all($pattern, $text, $matches);
print_r($matches[0]);

PHP

$text = "cat bat rat mat";
$pattern = '/(?<!b)at/';
preg_match_all($pattern, $text, $matches);
print_r($matches[0]);

A Array ( [0] => at [1] => at [2] => at )

B Array ( [0] => bat )

C Array ( [0] => at [1] => at )

D Array ( )

💡 Hint

Negative lookbehind excludes matches preceded by 'b'.

The pattern '(?

🔧 Debug

advanced

Identify the error in this lookbehind regex

This PHP code throws an error. What is the cause?

$pattern = '/(?<=\d+)abc/';
$text = '12abc';
preg_match($pattern, $text, $matches);

PHP

$pattern = '/(?<=\d+)abc/';
$text = '12abc';
preg_match($pattern, $text, $matches);

A Lookbehind pattern has variable length which is not supported

B Missing delimiter in regex pattern

C Unescaped backslash in pattern

D preg_match requires third argument to be passed by reference

💡 Hint

Lookbehind in PHP requires fixed length patterns.

PHP's PCRE engine requires lookbehind patterns to be of fixed length. '(?<=\d+)' is variable length (\d+ can match 1+ digits), which is not supported.

📝 Syntax

advanced

Which regex pattern uses both lookahead and lookbehind correctly?

Which of the following PHP regex patterns correctly matches 'foo' only if it is preceded by 'bar' and followed by 'baz'?

A /(?<!bar)foo(?!baz)/

B /(?<=bar)foo(?<=baz)/

C /(?=bar)foo(?=baz)/

D /(?<=bar)foo(?=baz)/

💡 Hint

Lookbehind checks before, lookahead checks after the match.

Option D uses positive lookbehind '(?<=bar)' to check before 'foo' and positive lookahead '(?=baz)' to check after 'foo'. This correctly matches 'foo' only when preceded by 'bar' and followed by 'baz'.

🚀 Application

expert

Count words not starting with a digit using lookahead

You want to count how many words in the string "$text = '1apple apple 2banana banana 3cherry cherry'" do not start with a digit. Which PHP code snippet correctly counts these words using lookahead?

PHP

$text = '1apple apple 2banana banana 3cherry cherry';
// Your code here

A

$pattern = '/(?<=\d)\b\w+\b/';
preg_match_all($pattern, $text, $matches);
echo count($matches[0]);

B

$pattern = '/\b(?!\d)\w+\b/';
preg_match_all($pattern, $text, $matches);
echo count($matches[0]);

C

$pattern = '/\b\w+\b(?<!\d)/';
preg_match_all($pattern, $text, $matches);
echo count($matches[0]);

D

$pattern = '/(?<!\d)\w+/';
preg_match_all($pattern, $text, $matches);
echo count($matches[0]);

💡 Hint

Use negative lookahead to exclude words starting with digits and word boundaries to match whole words.

Option B uses negative lookahead '(?!\d)' after '\b' to ensure the word does not start with a digit, and '\b\w+\b' to match whole words. This correctly counts 3 such words.