Preg_replace for substitution in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this preg_replace code?

Consider the following PHP code snippet using preg_replace. What will be printed?

PHP

<?php
$text = "The rain in Spain falls mainly in the plain.";
$result = preg_replace('/ain/', 'ane', $text);
echo $result;
?>

A The rane in Spane falls manely in the plane.

B The rain in Spain falls mainly in the plain.

C The rane in Spain falls mainly in the plain.

D The rane in Spain falls manely in the plain.

💡 Hint

Remember that preg_replace replaces all matches of the pattern.

The pattern '/ain/' matches every occurrence of 'ain' in the string. Each 'ain' is replaced by 'ane', so all 'ain' substrings become 'ane'.

❓ Predict Output

intermediate

What does this preg_replace code output?

What will be the output of this PHP code using preg_replace with a capturing group?

PHP

<?php
$input = "abc123def456";
$output = preg_replace('/(\d+)/', '<num>$1</num>', $input);
echo $output;
?>

A abc<num>123def456</num>

B abc<num>123</num>def<num>456</num>

C abc123def456

D abc<num></num>def<num></num>

💡 Hint

The pattern matches one or more digits and replaces each match with the digits wrapped in tags.

The regex '(\d+)' matches sequences of digits. Each sequence is replaced by the same digits wrapped in and tags. So '123' and '456' are replaced separately.

❓ Predict Output

advanced

What is the output of this preg_replace with limit?

What will this PHP code print when using preg_replace with a limit of 1 replacement?

PHP

<?php
$str = "one two three two one";
$result = preg_replace('/two/', '2', $str, 1);
echo $result;
?>

A one 2 three two one

B one 2 three 2 one

C one 2 three

D one two three two one

💡 Hint

The limit parameter controls how many replacements are made.

The pattern '/two/' matches 'two'. With limit 1, only the first occurrence is replaced by '2'. The rest remain unchanged.

❓ Predict Output

advanced

What error does this preg_replace code produce?

What error will this PHP code produce when run?

PHP

<?php
$text = "hello world";
$result = preg_replace('/(world/', 'earth', $text);
echo $result;
?>

A hello world

B hello earth

C Warning: preg_replace(): Compilation failed: missing ) at offset 7

D Fatal error: Uncaught Error: Call to undefined function preg_replace()

💡 Hint

Check the regex pattern for syntax errors.

The regex pattern '/(world/' is missing a closing parenthesis, causing a compilation error in preg_replace.

🧠 Conceptual

expert

How many replacements are made by this preg_replace call?

Given the PHP code below, how many replacements will preg_replace perform?

PHP

<?php
$str = "aaa bbb aaa bbb aaa";
$replaced = preg_replace('/aaa/', 'zzz', $str, -1, $count);
echo $count;
?>

A 5

B 1

C 0

D 3

💡 Hint

The fourth argument is the limit, and the fifth argument stores the count of replacements.

The pattern '/aaa/' matches 'aaa' three times in the string. The limit -1 means no limit, so all matches are replaced. The variable $count holds the number of replacements, which is 3.