Lookahead and lookbehind in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of positive lookahead regex in PHP

What is the output of this PHP code snippet?

$text = "apple pie and apple tart";
$pattern = '/apple(?= pie)/';
preg_match_all($pattern, $text, $matches);
print_r($matches[0]);

PHP

$text = "apple pie and apple tart";
$pattern = '/apple(?= pie)/';
preg_match_all($pattern, $text, $matches);
print_r($matches[0]);

A Array ( [0] => apple [1] => apple )

B Array ( [0] => pie )

C Array ( [0] => apple )

D Array ( )

💡 Hint

Lookahead checks what comes after the word without including it.

The pattern 'apple(?= pie)' matches 'apple' only if it is followed by ' pie'. In the text, only the first 'apple' is followed by ' pie', so only one match is found.

❓ Predict Output

intermediate

Output of negative lookbehind regex in PHP

What will this PHP code output?

$text = "cat bat rat mat";
$pattern = '/(?preg_match_all($pattern, $text, $matches);
print_r($matches[0]);

PHP

$text = "cat bat rat mat";
$pattern = '/(?<!b)at/';
preg_match_all($pattern, $text, $matches);
print_r($matches[0]);

A Array ( [0] => at [1] => at [2] => at )

B Array ( [0] => bat )

C Array ( [0] => at [1] => at )

D Array ( )

💡 Hint

Negative lookbehind excludes matches preceded by 'b'.

The pattern '(?

🔧 Debug

advanced

Identify the error in this lookbehind regex

This PHP code throws an error. What is the cause?

$pattern = '/(?<=\d+)abc/';
$text = '12abc';
preg_match($pattern, $text, $matches);

PHP

$pattern = '/(?<=\d+)abc/';
$text = '12abc';
preg_match($pattern, $text, $matches);

A Lookbehind pattern has variable length which is not supported

B Missing delimiter in regex pattern

C Unescaped backslash in pattern

D preg_match requires third argument to be passed by reference

💡 Hint

Lookbehind in PHP requires fixed length patterns.

PHP's PCRE engine requires lookbehind patterns to be of fixed length. '(?<=\d+)' is variable length (\d+ can match 1+ digits), which is not supported.

📝 Syntax

advanced

Which regex pattern uses both lookahead and lookbehind correctly?

Which of the following PHP regex patterns correctly matches 'foo' only if it is preceded by 'bar' and followed by 'baz'?

A /(?<!bar)foo(?!baz)/

B /(?<=bar)foo(?<=baz)/

C /(?=bar)foo(?=baz)/

D /(?<=bar)foo(?=baz)/

💡 Hint

Lookbehind checks before, lookahead checks after the match.

Option D uses positive lookbehind '(?<=bar)' to check before 'foo' and positive lookahead '(?=baz)' to check after 'foo'. This correctly matches 'foo' only when preceded by 'bar' and followed by 'baz'.

🚀 Application

expert

Count words not starting with a digit using lookahead

You want to count how many words in the string "$text = '1apple apple 2banana banana 3cherry cherry'" do not start with a digit. Which PHP code snippet correctly counts these words using lookahead?

PHP

$text = '1apple apple 2banana banana 3cherry cherry';
// Your code here

A

$pattern = '/(?<=\d)\b\w+\b/';
preg_match_all($pattern, $text, $matches);
echo count($matches[0]);

B

$pattern = '/\b(?!\d)\w+\b/';
preg_match_all($pattern, $text, $matches);
echo count($matches[0]);

C

$pattern = '/\b\w+\b(?<!\d)/';
preg_match_all($pattern, $text, $matches);
echo count($matches[0]);

D

$pattern = '/(?<!\d)\w+/';
preg_match_all($pattern, $text, $matches);
echo count($matches[0]);

💡 Hint

Use negative lookahead to exclude words starting with digits and word boundaries to match whole words.

Option B uses negative lookahead '(?!\d)' after '\b' to ensure the word does not start with a digit, and '\b\w+\b' to match whole words. This correctly counts 3 such words.

Practice

1. What does a positive lookahead (?=pattern) do in PHP regular expressions?

easy

A. Matches the pattern and consumes it in the string

B. Checks if the specified pattern precedes the current position and includes it in the match

C. Checks if the specified pattern follows the current position without including it in the match

D. Checks if the pattern is anywhere in the string regardless of position

Solution

1. Step 1: Understand lookahead behavior

   A positive lookahead (?=pattern) checks if the pattern appears immediately after the current position without consuming characters.

2. Step 2: Compare options with definition

   Checks if the specified pattern follows the current position without including it in the match correctly describes this behavior. Options B and C incorrectly describe lookbehind or normal matching. Checks if the pattern is anywhere in the string regardless of position is too broad.

3. Final Answer:

   Checks if the specified pattern follows the current position without including it in the match -> Option C

4. Quick Check:

   Positive lookahead = check after position [OK]

Hint: Lookahead checks after current spot without consuming [OK]

Common Mistakes:

• Confusing lookahead with lookbehind
• Thinking lookahead consumes characters
• Assuming it matches anywhere, not just after position

2. Which of the following is the correct syntax for a negative lookbehind in PHP regex?

easy

A. (?pattern)

B. (?<=pattern)

C. (?!pattern)

D. (?=pattern)

Solution

1. Step 1: Recall lookbehind syntax

   Negative lookbehind in PHP regex is written as (?<!pattern), which asserts that the pattern does NOT appear before the current position.

2. Step 2: Match options to syntax

   (?

3. Final Answer:

   (?<!pattern) -> Option A

4. Quick Check:

   Negative lookbehind = (?<!pattern) [OK]

Hint: Lookbehind uses (?<=) or (?), negative has exclamation [OK]

Common Mistakes:

• Mixing lookahead and lookbehind syntax
• Using wrong brackets or missing exclamation mark
• Confusing negative and positive forms

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

$text = "apple pie";
if (preg_match('/apple(?= pie)/', $text)) {
    echo "Match found";
} else {
    echo "No match";
}

medium

A. Empty output

B. Match found

C. Syntax error

D. No match

Solution

1. Step 1: Understand the regex pattern

   The pattern /apple(?= pie)/ matches 'apple' only if it is immediately followed by ' pie' without including ' pie' in the match.

2. Step 2: Check the input string

   The string is "apple pie", so 'apple' is followed by ' pie'. The lookahead condition is true, so the match succeeds.

3. Final Answer:

   Match found -> Option B

4. Quick Check:

   Lookahead confirms 'apple' before ' pie' = Match found [OK]

Hint: Lookahead checks next chars without consuming [OK]

Common Mistakes:

• Thinking lookahead consumes characters
• Assuming no match because ' pie' is not part of match
• Confusing output with syntax error

4. Identify the error in this PHP regex using lookbehind:

$text = "hello world";
if (preg_match('/(?

medium

A. No error, code runs fine

B. Lookbehind syntax is incorrect, missing brackets

C. Lookbehind must be at the end of the pattern

D. Lookbehind pattern is fixed width but 'hello' is variable length

Solution

1. Step 1: Check lookbehind syntax and pattern

   The lookbehind (?<!hello) is valid because 'hello' is fixed length (5 characters), which PHP supports.

2. Step 2: Analyze the string and pattern match

   The string is "hello world". The lookbehind asserts 'world' is NOT preceded by 'hello'. Here, 'world' is preceded by 'hello', so the match fails and "No match" is printed.

3. Final Answer:

   No error, code runs fine -> Option A

4. Quick Check:

   Lookbehind syntax correct, no error [OK]

Hint: Lookbehind must be fixed width, 'hello' is fixed length [OK]

Common Mistakes:

• Thinking 'hello' is variable length
• Assuming lookbehind syntax is wrong
• Expecting match when lookbehind condition fails

5. You want to match all occurrences of the word 'cat' only if it is NOT preceded by 'wild' and is immediately followed by 's' in a string. Which regex pattern correctly uses lookahead and lookbehind in PHP?

hard

A. /(?<=wild )cat(?=s)/

B. /(?<=wild)cat(?=s)/

C. /(?wild)cat(?= s)/

D. /(?wild )cat(?=s)/

Solution

1. Step 1: Understand the requirements

   We want 'cat' that is NOT preceded by 'wild ' (note the space) and IS followed immediately by 's'.

2. Step 2: Analyze each option

   /(?(?<!wild ) to exclude 'wild ' before 'cat' and positive lookahead (?=s) to ensure 'cat' is followed by 's'. This matches the requirement.
   /(?<=wild )cat(?=s)/ uses positive lookbehind, which requires 'wild ' before 'cat', opposite of requirement.
   /(?/(?<=wild)cat(?=s)/ uses positive lookbehind without space, which is incorrect.

3. Final Answer:

   /(?<!wild )cat(?=s)/ -> Option D

4. Quick Check:

   Negative lookbehind + positive lookahead = /(?

Hint: Negative lookbehind excludes prefix, positive lookahead confirms suffix [OK]

Common Mistakes:

• Forgetting space after 'wild' in lookbehind
• Using positive lookbehind instead of negative
• Adding space in lookahead incorrectly