Preg_match_all for global matching in PHP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find all matches of 'cat' in the string.

PHP

<?php
$subject = "The cat sat on the cat mat.";
preg_match_all([1], $subject, $matches);
print_r($matches[0]);
?>

Drag options to blanks, or click blank then click option'

A"/cat/i"

B"cat"

C"/cat/"

D"/cat/g"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the pattern without slashes causes an error.

Adding 'g' modifier is invalid in PHP regex.

The pattern must be enclosed in slashes to be a valid regex in PHP. So "/cat/" is correct.

2fill in blank

medium

Complete the code to perform a case-insensitive global match for 'dog'.

PHP

<?php
$subject = "Dog dog DOG doG";
preg_match_all([1], $subject, $matches);
print_r($matches[0]);
?>

Drag options to blanks, or click blank then click option'

A"/dog/i"

B"/dog/"

C"dog"

D"/dog/g"

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'g' modifier which is invalid in PHP.

Not using slashes around the pattern.

The 'i' modifier makes the match case-insensitive. PHP does global matching by default with preg_match_all.

3fill in blank

hard

Fix the error in the code to correctly find all numbers in the string.

PHP

<?php
$subject = "123 abc 456 def 789";
preg_match_all([1], $subject, $matches);
print_r($matches[0]);
?>

Drag options to blanks, or click blank then click option'

A"/\d+/"

B"/d+/"

C"/\d*/"

D"/\D+/"

Attempts:

3 left

💡 Hint

Common Mistakes

Not escaping the backslash causes an error.

Using '\D' matches non-digits, which is wrong.

The pattern "/\d+/" matches one or more digits. The backslash must be escaped in PHP strings.

4fill in blank

hard

Fill both blanks to find all words starting with 'a' or 'A' in the string.

PHP

<?php
$subject = "Apple and apricot are awesome.";
preg_match_all([1], $subject, $matches, [2]);
print_r($matches[0]);
?>

Drag options to blanks, or click blank then click option'

A"/\b[aA]\w*/"

BPREG_OFFSET_CAPTURE

CPREG_PATTERN_ORDER

DPREG_SET_ORDER

Attempts:

3 left

💡 Hint

Common Mistakes

Using PREG_SET_ORDER changes the structure of matches.

Not escaping backslashes in the pattern.

The pattern matches words starting with 'a' or 'A'. PREG_PATTERN_ORDER organizes matches by pattern order.

5fill in blank

hard

Fill all three blanks to extract all email addresses from the text.

PHP

<?php
$text = "Contact us at [e-mail] or [e-mail].";
preg_match_all([1], $text, $matches, [2], [3]);
print_r($matches[0]);
?>

Drag options to blanks, or click blank then click option'

A"/[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}/"

B0

CPREG_PATTERN_ORDER

DPREG_OFFSET_CAPTURE

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong offset or flags changes output structure.

Incorrect regex pattern misses valid emails.

The regex matches emails. Offset 0 means start at beginning. PREG_PATTERN_ORDER organizes matches by pattern.

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