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.

Practice

1. What does the PHP function preg_replace do?

easy

A. It splits a string into an array based on a pattern.

B. It counts how many times a pattern appears in a string.

C. It finds text matching a pattern and replaces it with new text.

D. It checks if a string matches a pattern and returns true or false.

Solution

1. Step 1: Understand the purpose of preg_replace

   The function preg_replace searches a string for parts that match a pattern and replaces those parts with new text.

2. Step 2: Compare with other functions

   Counting matches or splitting strings are done by other functions like preg_match_all or preg_split, not preg_replace.

3. Final Answer:

   It finds text matching a pattern and replaces it with new text. -> Option C

4. Quick Check:

   preg_replace replaces matched text [OK]

Hint: Remember: replace means find and change text [OK]

Common Mistakes:

• Confusing preg_replace with preg_match
• Thinking preg_replace counts matches
• Mixing preg_replace with string splitting functions

2. Which of the following is the correct syntax to replace all digits with '#' in PHP using preg_replace?

easy

A. preg_replace('/\D/', '#', $text);

B. preg_replace('/d/', '#', $text);

C. preg_replace('/[0-9]+/', '#', $text);

D. preg_replace('/\d/', '#', $text);

Solution

1. Step 1: Identify the pattern for digits

   The pattern '\d' matches any single digit (0-9). In PHP single-quoted strings, it is written as '/\d/'.

2. Step 2: Check other options

   B uses '/d/', which matches the literal letter 'd'. C uses '/[0-9]+/', which matches one or more consecutive digits, replacing the entire group with a single '#'. D uses '/\D/', which matches non-digits.

3. Final Answer:

   preg_replace('/\d/', '#', $text); -> Option D

4. Quick Check:

   Use '\d' to match digits [OK]

Hint: Use '\d' to match digits in regex [OK]

Common Mistakes:

• Forgetting to escape backslash in PHP strings
• Using 'd' instead of '\d' for digits
• Confusing '\d' with '\D' (non-digits)

3. What will be the output of the following PHP code?

$text = "Hello 123 World";
$result = preg_replace('/\d+/', 'NUM', $text);
echo $result;

medium

A. Hello NUM World

B. Hello 123 World

C. NUM NUM NUM World

D. Hello NUM123 World

Solution

1. Step 1: Understand the regex pattern '/\d+/'

   The pattern '\d+' matches one or more digits in a row. In the string, '123' matches this pattern.

2. Step 2: Apply preg_replace to the string

   The matched digits '123' are replaced by 'NUM'. So the string becomes 'Hello NUM World'.

3. Final Answer:

   Hello NUM World -> Option A

4. Quick Check:

   Digits replaced by 'NUM' = Hello NUM World [OK]

Hint: Look for digit groups replaced by 'NUM' [OK]

Common Mistakes:

• Thinking only single digits replaced separately
• Expecting original digits to remain
• Confusing pattern with non-digit matching

4. Identify the error in this PHP code snippet:

$text = "abc123def";
$result = preg_replace('/[a-z]/', 1, $text, 2);
echo $result;

medium

A. The limit parameter should be the third argument, not the fourth.

B. The replacement value must be a string, not an integer.

C. The pattern should be enclosed in double quotes, not single quotes.

D. preg_replace cannot limit replacements; the code is correct.

Solution

1. Step 1: Check preg_replace argument order

   The arguments are: pattern, replacement, subject, limit. The limit is correctly the fourth argument.

2. Step 2: Check replacement type

   The replacement should be a string. Using integer 1 will be converted to string '1' but is not recommended and can cause confusion.

3. Step 3: Check other options

   Pattern can be in single quotes. preg_replace supports limit parameter.

4. Final Answer:

   The replacement value must be a string, not an integer. -> Option B

5. Quick Check:

   Replacement should be string [OK]

Hint: Always use string for replacement in preg_replace [OK]

Common Mistakes:

• Passing integer instead of string as replacement
• Misplacing the limit parameter
• Thinking preg_replace can't limit replacements

5. You want to replace only the first two occurrences of the word 'apple' with 'orange' in the string 'apple banana apple apple grape'. Which preg_replace call achieves this?

hard

A. preg_replace('/apple/', 'orange', 'apple banana apple apple grape', 2);

B. preg_replace('/apple/', 'orange', 'apple banana apple apple grape');

C. preg_replace('/apple/', 'orange', 'apple banana apple apple grape', 3);

D. preg_replace('/apple/', 'orange', 'apple banana apple apple grape', 1);

Solution

1. Step 1: Understand the limit parameter in preg_replace

   The fourth argument limits how many replacements happen. To replace only first two 'apple', limit should be 2.

2. Step 2: Check each option's limit

   preg_replace('/apple/', 'orange', 'apple banana apple apple grape', 2); uses limit 2, so only first two 'apple' replaced. preg_replace('/apple/', 'orange', 'apple banana apple apple grape'); replaces all. preg_replace('/apple/', 'orange', 'apple banana apple apple grape', 3); replaces first three. preg_replace('/apple/', 'orange', 'apple banana apple apple grape', 1); replaces only first one.

3. Final Answer:

   preg_replace('/apple/', 'orange', 'apple banana apple apple grape', 2); -> Option A

4. Quick Check:

   Limit 2 replaces first two matches [OK]

Hint: Use limit parameter to control replacements count [OK]

Common Mistakes:

• Omitting limit to replace all occurrences
• Setting wrong limit number
• Confusing order of arguments