Preg_match_all for global matching in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of preg_match_all with a simple pattern?

Consider the following PHP code snippet using preg_match_all. What will be the output?

PHP

<?php
$subject = 'apple banana apple cherry';
$pattern = '/apple/';
preg_match_all($pattern, $subject, $matches);
print_r($matches[0]);
?>

A Array ( [0] => apple )

B Array ( [0] => apple banana apple cherry )

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

D Array ( )

💡 Hint

preg_match_all finds all matches, not just the first one.

The pattern '/apple/' matches the word 'apple' twice in the string. preg_match_all collects all matches in $matches[0].

❓ Predict Output

intermediate

How many matches does preg_match_all find?

Given this PHP code, how many matches will preg_match_all find?

PHP

<?php
$subject = 'cat bat rat mat';
$pattern = '/.at/';
$count = preg_match_all($pattern, $subject, $matches);
echo $count;
?>

A 4

B 3

C 1

D 0

💡 Hint

The pattern matches any character followed by 'at'.

The pattern '/.at/' matches 'cat', 'bat', 'rat', and 'mat' — four matches total.

🔧 Debug

advanced

Why does this preg_match_all code produce an empty array?

Look at this PHP code. Why does preg_match_all return no matches?

PHP

<?php
$subject = '123-456-7890';
$pattern = '/\d{3}-\d{3}-\d{4}/';
preg_match_all($pattern, $subject, $matches);
print_r($matches[0]);
?>

A It returns an empty array because the pattern is correct but preg_match_all expects global flag 'g' which PHP does not use.

B It returns an empty array because the pattern is missing delimiters.

C It returns an empty array because the subject string does not contain the pattern.

D It returns an empty array because the backslashes are not escaped properly in the pattern string.

💡 Hint

Check how backslashes are used inside PHP strings for regex.

In PHP, backslashes must be escaped in double-quoted strings. The pattern should be '/\\d{3}-\\d{3}-\\d{4}/' or use single quotes to avoid double escaping.

📝 Syntax

advanced

Which option causes a syntax error in preg_match_all usage?

Identify which PHP code snippet will cause a syntax error when using preg_match_all.

A preg_match_all('/foo/', $text, $matches, 'PREG_OFFSET_CAPTURE');

B preg_match_all('/foo/', $text, $matches, PREG_OFFSET_CAPTURE);

C preg_match_all('/foo/', $text, $matches);

D preg_match_all('/foo/', $text, $matches, 0);

💡 Hint

Check the type of the fourth argument in preg_match_all.

The fourth argument must be an integer flag, not a string. Using 'PREG_OFFSET_CAPTURE' as a string causes a syntax error.

🚀 Application

expert

Extract all email addresses from a text using preg_match_all

You want to extract all email addresses from a string using preg_match_all. Which code snippet correctly captures all emails?

A

$pattern = '/[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}/g';
preg_match_all($pattern, $text, $matches);

B

$pattern = '/[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}/';
preg_match_all($pattern, $text, $matches);

C

$pattern = '/[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}/i';
preg_match_all($pattern, $text, $matches);

D

$pattern = '/[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}/';
preg_match($pattern, $text, $matches);

💡 Hint

Remember PHP regex does not use 'g' flag and preg_match_all finds all matches.

Option B uses the correct pattern and preg_match_all to find all emails. Option B uses 'g' which is invalid in PHP. Option B adds 'i' for case-insensitive but is optional. Option B uses preg_match which finds only the first match.