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.

Practice

1. What does the PHP function preg_match_all do?

easy

A. Splits a string into an array using a pattern.

B. Finds only the first match of a pattern in a string.

C. Replaces all matches of a pattern in a string with another string.

D. Finds all matches of a pattern in a string and stores them in an array.

Solution

1. Step 1: Understand the function purpose

   preg_match_all is designed to find every occurrence of a pattern in a string, not just the first one.

2. Step 2: Compare with other functions

   preg_match finds only the first match, preg_replace replaces matches, and preg_split splits strings. So only preg_match_all finds all matches and stores them.

3. Final Answer:

   Finds all matches of a pattern in a string and stores them in an array. -> Option D

4. Quick Check:

   preg_match_all = all matches found [OK]

Hint: preg_match_all finds every match, not just one [OK]

Common Mistakes:

• Confusing preg_match_all with preg_match
• Thinking it replaces matches
• Assuming it splits strings

2. Which of the following is the correct syntax to use preg_match_all in PHP?

easy

A. preg_match_all($subject, $pattern, $matches);

B. preg_match_all($pattern, $subject, $matches);

C. preg_match_all($pattern, $matches, $subject);

D. preg_match_all($matches, $pattern, $subject);

Solution

1. Step 1: Recall function parameter order

   The correct order is pattern first, then the string to search (subject), then the array to store matches.

2. Step 2: Verify options

   Only preg_match_all($pattern, $subject, $matches); follows this order: preg_match_all($pattern, $subject, $matches);

3. Final Answer:

   preg_match_all($pattern, $subject, $matches); -> Option B

4. Quick Check:

   Pattern, subject, matches array order [OK]

Hint: Pattern comes first, then string, then matches array [OK]

Common Mistakes:

• Swapping pattern and subject parameters
• Placing matches array before subject
• Using wrong parameter order

3. What will be the output of the following PHP code?

$text = "apple, banana, cherry";
preg_match_all('/\b\w{6}\b/', $text, $matches);
print_r($matches[0]);

medium

A. ["apple"]

B. ["apple", "banana", "cherry"]

C. ["banana", "cherry"]

D. []

Solution

1. Step 1: Understand the regex pattern

   The pattern /\b\w{6}\b/ matches words with exactly 6 letters.

2. Step 2: Check the words in the string

   "apple" has 5 letters, "banana" has 6, "cherry" has 6. So only "banana" and "cherry" match.

3. Final Answer:

   ["banana", "cherry"] -> Option C

4. Quick Check:

   Words with 6 letters = banana, cherry [OK]

Hint: Count letters to match \w{6} exactly [OK]

Common Mistakes:

• Including words with length not equal to 6
• Misunderstanding \b word boundaries
• Confusing \w with any character

4. Identify the error in this PHP code snippet using preg_match_all:

$text = "one two three";
preg_match_all('/\w+/', $matches, $text);
print_r($matches);

medium

A. The order of parameters is incorrect.

B. The regex pattern is invalid.

C. The variable $matches is not initialized.

D. The print_r function cannot print arrays.

Solution

1. Step 1: Check parameter order in preg_match_all

   The correct order is pattern, subject string, then matches array. Here, $matches and $text are swapped.

2. Step 2: Identify the impact

   Passing $matches as the subject and $text as matches causes wrong behavior and no matches found.

3. Final Answer:

   The order of parameters is incorrect. -> Option A

4. Quick Check:

   Parameter order matters in preg_match_all [OK]

Hint: Remember: pattern, subject, matches array order [OK]

Common Mistakes:

• Swapping subject and matches parameters
• Assuming regex pattern is wrong
• Thinking print_r can't print arrays

5. You want to extract all email addresses from a text using preg_match_all. Which pattern correctly matches a simple email format (letters, digits, dots, underscores before @, then domain)?

hard

A. '/[\w\.]+@[\w\.]+\.[a-z]{2,}/i'

B. '/\d+@[a-z]+\.[a-z]{2,}/'

C. '/[a-z]+@[a-z]+\.[a-z]{3}/'

D. '/\w+\s+@\s+\w+\.com/'

Solution

1. Step 1: Analyze the email pattern requirements

   Emails have letters, digits, dots, underscores before @, then domain with dot and extension.

2. Step 2: Check each pattern

   '/[\w\.]+@[\w\.]+\.[a-z]{2,}/i' matches word characters and dots before @, then domain and extension with at least 2 letters, case insensitive. Others are too restrictive or incorrect.

3. Final Answer:

   '/[\w\.]+@[\w\.]+\.[a-z]{2,}/i' -> Option A

4. Quick Check:

   Pattern includes letters, digits, dots, underscores [OK]

Hint: Use \w and dot before @, domain with dot and 2+ letters [OK]

Common Mistakes:

• Using \d+ only before @ (digits only)
• Forcing exact 3-letter domain extensions
• Adding spaces inside email pattern