Capturing groups and backreferences in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PHP code using capturing groups?

Consider the following PHP code that uses a regular expression with capturing groups. What will it output?

PHP

<?php
$subject = "apple banana apple";
$pattern = '/(apple) (banana) \1/';
if (preg_match($pattern, $subject, $matches)) {
    echo $matches[0];
} else {
    echo "No match";
}
?>

A apple banana apple

B apple banana banana

C No match

D banana apple banana

💡 Hint

Remember that \1 refers to the first captured group in the pattern.

The pattern matches 'apple banana apple' because \1 matches the first captured group 'apple'. So the full match is 'apple banana apple'.

❓ Predict Output

intermediate

What does this PHP code output with nested capturing groups?

Look at this PHP code using nested capturing groups and backreferences. What will it print?

PHP

<?php
$subject = "abcabc";
$pattern = '/(abc)(\1)/';
if (preg_match($pattern, $subject, $matches)) {
    echo count($matches);
} else {
    echo "No match";
}
?>

A No match

B 3

C 2

D 4

💡 Hint

Count how many capturing groups are matched and stored in $matches.

The pattern has two capturing groups: (abc) and (\1). The second group matches the first group's content again. So $matches has 3 elements: full match, group 1, group 2.

🔧 Debug

advanced

Why does this PHP regex fail to match?

This PHP code tries to match repeated words using backreferences but fails. What is the cause?

PHP

<?php
$subject = "test test";
$pattern = "/(\w+) \1/";
if (preg_match($pattern, $subject)) {
    echo "Matched!";
} else {
    echo "No match";
}
?>

A Backreference \1 is not recognized because it needs double escaping in PHP strings.

B The pattern misses a quantifier to repeat the word.

C The subject string has extra spaces causing mismatch.

D The pattern is correct and will match 'test test'.

💡 Hint

Check how PHP interprets backslashes inside single-quoted vs double-quoted strings.

In PHP double-quoted strings, backslashes need to be escaped. The pattern uses '\1' but inside double quotes it becomes '\1' which is interpreted as an octal escape. It should be '\\1' to represent a backreference.

📝 Syntax

advanced

Which option causes a syntax error in PHP regex with capturing groups?

Identify which PHP regex pattern will cause a syntax error when used in preg_match.

A '/(foo|bar)/'

B '/(foo)(bar)/'

C '/(foo)\1/'

D '/(foo(bar)/'

💡 Hint

Look for unbalanced parentheses in the pattern.

Option D has unbalanced parentheses: '(foo(bar)' is missing a closing parenthesis, causing a regex syntax error.

🚀 Application

expert

How many matches does this PHP code find using backreferences?

This PHP code uses preg_match_all with a pattern that uses backreferences. How many matches will it find?

PHP

<?php
$subject = "cat cat dog dog cat";
$pattern = '/(\w+) \1/';
preg_match_all($pattern, $subject, $matches);
echo count($matches[0]);
?>

A 1

B 3

C 2

D 0

💡 Hint

Look for repeated words separated by a space in the subject string.

The pattern matches repeated words separated by a space. 'cat cat' and 'dog dog' match, but 'cat' at the end is alone. So there are 2 matches.