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'.