Preg_match_all for global matching in PHP - Mini Project: Build & Apply

Using preg_match_all for Global Matching in PHP

📖 Scenario: You have a paragraph of text and want to find all the words that start with the letter 'a'.

🎯 Goal: Build a PHP script that uses preg_match_all to find all words starting with 'a' in a given text.

📋 What You'll Learn

Create a string variable called text with the exact sentence: "An apple a day keeps the doctor away."

Create a variable called pattern that holds the regex pattern to find words starting with 'a' (case-insensitive).

Use preg_match_all with pattern and text to find all matches and store them in matches.

Print the array matches[0] which contains all matched words.

💡 Why This Matters

🌍 Real World

Finding all occurrences of a pattern in text is useful in text analysis, search engines, and data extraction.

💼 Career

Many programming jobs require working with text data and using regular expressions to extract information efficiently.

1

DATA SETUP: Create the text string

Create a string variable called text and set it to the exact sentence: "An apple a day keeps the doctor away."

PHP

# Your code here

Hint

Use double quotes to create the string exactly as shown.

2

CONFIGURATION: Create the regex pattern

Create a variable called pattern and set it to the regex pattern "/\ba\w*/i" to find words starting with 'a' (case-insensitive).

PHP

<?php
$text = "An apple a day keeps the doctor away.";
# Your code here

Hint

The pattern /\ba\w*/i means words starting with 'a' ignoring case.

3

CORE LOGIC: Use preg_match_all to find matches

Use preg_match_all with pattern and text to find all matches and store them in a variable called matches.

PHP

<?php
$text = "An apple a day keeps the doctor away.";
$pattern = "/\ba\w*/i";
# Your code here

Hint

Use preg_match_all with the pattern, text, and matches variables.

4

OUTPUT: Print the matched words

Print the array matches[0] which contains all matched words using print_r.

PHP

<?php
$text = "An apple a day keeps the doctor away.";
$pattern = "/\ba\w*/i";
preg_match_all($pattern, $text, $matches);
# Your code here

Hint

Use print_r($matches[0]) to display the matched words array.

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