Lookahead and lookbehind in PHP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to match 'foo' only if it is followed by 'bar'.

PHP

$pattern = '/foo[1]bar/';

Drag options to blanks, or click blank then click option'

A(?=bar)

B(?=)

C(?<=bar)

D(?!bar)

Attempts:

3 left

💡 Hint

Common Mistakes

Using '(?<=bar)' which is a lookbehind, not lookahead.

Using '(?!bar)' which is a negative lookahead.

Leaving the lookahead empty as '(?=)'.

The syntax '(?=bar)' is a positive lookahead that ensures 'foo' is followed by 'bar'.

2fill in blank

medium

Complete the code to match 'bar' only if it is preceded by 'foo'.

PHP

$pattern = '/[1]bar/';

Drag options to blanks, or click blank then click option'

A(?=foo)

B(?!foo)

C(?<!foo)

D(?<=foo)

Attempts:

3 left

💡 Hint

Common Mistakes

Using '(?=foo)' which is a lookahead, not lookbehind.

Using '(?

Using '(?!foo)' which is a negative lookahead.

The syntax '(?<=foo)' is a positive lookbehind that ensures 'bar' is preceded by 'foo'.

3fill in blank

hard

Fix the error in the pattern to match 'cat' only if it is not followed by 'dog'.

PHP

$pattern = '/cat[1]dog/';

Drag options to blanks, or click blank then click option'

A(?!dog)

B(?<!dog)

C(?<=dog)

D(?=)

Attempts:

3 left

💡 Hint

Common Mistakes

Using '(?

Using '(?=dog)' which is a positive lookahead.

Leaving the lookahead empty as '(?=)'.

The syntax '(?!dog)' is a negative lookahead that ensures 'cat' is not followed by 'dog'.

4fill in blank

hard

Fill both blanks to match 'apple' only if it is preceded by 'green' and not followed by 'red'.

PHP

$pattern = '/[1]apple[2]/';

Drag options to blanks, or click blank then click option'

A(?<=green)

B(?=red)

C(?!red)

D(?<!green)

Attempts:

3 left

💡 Hint

Common Mistakes

Using '(?=red)' instead of negative lookahead.

Using '(?

Mixing up lookahead and lookbehind positions.

Use '(?<=green)' for positive lookbehind and '(?!red)' for negative lookahead.

5fill in blank

hard

Fill all three blanks to create a pattern that matches 'dog' only if it is preceded by 'big' or 'fat' and not followed by 'cat'.

PHP

$pattern = '/(?<=([1]|[2]))dog[3]cat/';

Drag options to blanks, or click blank then click option'

Abig

Bfat

C(?!

D(?<!

Attempts:

3 left

💡 Hint

Common Mistakes

Using '(?

Not using parentheses around alternatives in lookbehind.

Mixing up the order of lookbehind and lookahead.

Use '(?<=big|fat)' for lookbehind and '(?!cat)' for negative lookahead after 'dog'.

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