Preg_match_all for global matching in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does preg_match_all do in PHP?

preg_match_all finds all matches of a pattern in a string, not just the first one. It returns the number of matches and stores all matches as an array in a variable passed by reference.

beginner

What is the difference between preg_match and preg_match_all?

preg_match finds only the first match of a pattern in a string. preg_match_all finds all matches throughout the string.

beginner

How do you access all matches found by preg_match_all?

Matches are stored in an array passed by reference as the third argument. You can loop through this array to use each match.

beginner

What does the return value of preg_match_all represent?

The return value is the number of full pattern matches found in the string. If no matches, it returns 0.

beginner

Example: What will preg_match_all('/\d+/', 'I have 2 apples and 10 oranges.', $matches); find?

It will find all numbers: 2 and 10. These will be stored in $matches[0] as ['2', '10'].

What does preg_match_all return if no matches are found?

A 0

B false

C null

D 1

preg_match_all returns 0 when no matches are found.

Which argument of preg_match_all holds the matches?

A Second argument

B Third argument

C First argument

D Fourth argument

The third argument is the variable passed by reference to store all matches.

If you want to find only the first match, which function should you use?

A preg_split

B preg_match_all

C preg_replace

D preg_match

preg_match finds only the first match.

What type of data does preg_match_all return in the matches array?

A String

B Integer

C Array of strings

D Boolean

Matches are returned as an array of strings.

Which pattern will match all words in a string using preg_match_all?

A /\w+/

B /\d+/

C /\s+/

D /[aeiou]/

/\w+/ matches sequences of word characters (letters, digits, underscore).

Explain how preg_match_all works and how you use it to find all matches in a string.

Think about how you get all results, not just one.

Describe the difference between preg_match and preg_match_all with an example.

Focus on how many matches each function returns.

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