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.

Practice

1. What does a capturing group in a PHP regular expression do?

easy

A. It ignores the matched text inside parentheses.

B. It repeats the entire pattern multiple times.

C. It saves part of the matched text for later use.

D. It changes the case of matched text.

Solution

1. Step 1: Understand capturing groups

   Parentheses () in regex save the matched part for reuse.

2. Step 2: Purpose of capturing groups

   This saved part can be accessed later in the code or reused in the pattern.

3. Final Answer:

   It saves part of the matched text for later use. -> Option C

4. Quick Check:

   Capturing group = save matched text [OK]

Hint: Parentheses capture text to reuse later in regex or code [OK]

Common Mistakes:

• Thinking capturing groups repeat patterns
• Believing they ignore matched text
• Assuming they change text case

2. Which of the following is the correct syntax for a backreference to the first capturing group in a PHP regex pattern?

easy

A. \(1\)

B. \\1

C. \$1

D. \1

Solution

1. Step 1: Identify backreference syntax in PHP regex

   Backreferences use a single backslash followed by group number, like \1 in regex patterns.

2. Step 2: Differentiate from string escaping

   In PHP strings, to represent \1 you write "\\1" but the regex pattern itself uses \1.

3. Final Answer:

   \1 -> Option D

4. Quick Check:

   Backreference first group = \1 [OK]

Hint: Use single backslash before group number for backreference in regex [OK]

Common Mistakes:

• Using double backslash instead of single
• Confusing backreference with variable syntax
• Adding parentheses around the number

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

$pattern = '/(\w+)\s\1/';
$subject = 'hello hello world';
if (preg_match($pattern, $subject, $matches)) {
    echo $matches[0];
} else {
    echo 'No match';
}

medium

A. hello world

B. hello hello

C. No match

D. hello

Solution

1. Step 1: Understand the regex pattern

   The pattern /(\w+)\s\1/ matches a word, a space, then the same word again (backreference \1).

2. Step 2: Apply pattern to the subject string

   The string 'hello hello world' contains 'hello hello' which matches the pattern.

3. Final Answer:

   hello hello -> Option B

4. Quick Check:

   Backreference matches repeated word = hello hello [OK]

Hint: Backreference \1 matches exact previous captured word [OK]

Common Mistakes:

• Ignoring backreference meaning
• Matching different words instead of repeated
• Expecting full string match

4. Identify the error in this PHP regex pattern: preg_match('/(\d+)\s\2/', '123 123', $matches);

medium

A. Backreference \2 is invalid because only one group is captured.

B. The pattern should use \1 instead of \2 for backreference.

C. The pattern is missing delimiters.

D. The pattern should not use capturing groups.

Solution

1. Step 1: Count capturing groups in the pattern

   There is only one capturing group: (\d+), so only \1 is valid.

2. Step 2: Check backreference usage

   Using \2 refers to a second group which does not exist, causing an error.

3. Final Answer:

   Backreference \2 is invalid because only one group is captured. -> Option A

4. Quick Check:

   Backreference number must match captured groups [OK]

Hint: Backreference number must not exceed captured groups count [OK]

Common Mistakes:

• Using backreference number higher than groups
• Ignoring missing groups
• Confusing delimiters with backreferences

5. Given the string 'abc123abc456', which regex pattern using capturing groups and backreferences will match the repeated substring 'abc' only if it appears twice separated by digits?

hard

A. /(abc)\d+\1/

B. /(abc)\1\d+/

C. /abc(\d+)abc/

D. /(abc)+\d+/

Solution

1. Step 1: Understand the requirement

   We want to match 'abc' repeated twice with digits in between.

2. Step 2: Analyze each pattern

   /(abc)\d+\1/: /(abc)\d+\1/ matches 'abc', digits, then same 'abc' again. This fits the requirement.
   /(abc)\1\d+/: /(abc)\1\d+/ expects 'abcabc' then digits, which is incorrect.
   /abc(\d+)abc/: /abc(\d+)abc/ matches 'abc', digits, 'abc' but digits are captured, not 'abc'.
   /(abc)+\d+/: /(abc)+\d+/ matches one or more 'abc' then digits, but no backreference to ensure repetition.

3. Final Answer:

   /(abc)\d+\1/ -> Option A

4. Quick Check:

   Backreference \1 repeats captured 'abc' after digits [OK]

Hint: Backreference \1 repeats first group exactly where needed [OK]

Common Mistakes:

• Placing backreference before digits
• Not using backreference to repeat 'abc'
• Capturing digits instead of 'abc'