Preg_replace for substitution in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the preg_replace function do in PHP?

preg_replace searches a string for matches to a regular expression pattern and replaces them with a specified replacement string.

beginner

What are the three main parameters of preg_replace?

• pattern: The regex pattern to search for.
• replacement: The string to replace matches with.
• subject: The input string to search and replace in.

beginner

How do you use preg_replace to replace all digits in a string with a dash (-)?

preg_replace('/\d/', '-', 'Phone: 123-456'); // Result: 'Phone: --- ---'

beginner

What happens if no matches are found when using preg_replace?

The original string is returned unchanged because there is nothing to replace.

intermediate

Can preg_replace use backreferences in the replacement string? Give an example.

<p>Yes. Backreferences let you reuse parts of the matched text in the replacement.</p><pre>preg_replace('/(\w+) (\w+)/', '$2 $1', 'John Doe'); // Result: 'Doe John'</pre>

What does preg_replace('/cat/', 'dog', 'catapult') return?

A catapult

B catdog

C dog

D dogapult

The pattern 'cat' is found at the start of 'catapult' and replaced with 'dog', resulting in 'dogapult'.

Which delimiter is used in the pattern /\d+/ in preg_replace?

A #

B /

C ~

D %

The slashes / are the delimiters that mark the start and end of the regex pattern.

What will preg_replace('/(\w+) (\w+)/', '$2 $1', 'Hello World') output?

A $2 $1

B Hello World

C World Hello

D Hello

The two words are swapped using backreferences $2 and $1 in the replacement string.

If the pattern does not match anything in the subject, what does preg_replace return?

A The original subject string

B NULL

C An error

D An empty string

No matches means no replacements, so the original string is returned unchanged.

Which of these is a valid use of preg_replace to remove all vowels from a string?

A preg_replace('/[aeiou]/i', '', 'Hello')

B preg_replace('aeiou', '', 'Hello')

C preg_replace('/aeiou/', '', 'Hello')

D preg_replace('/[aeiou]/', ' ', 'Hello')

Option A uses a regex pattern with character class and case-insensitive flag i to remove vowels.

Explain how preg_replace works for substituting text in a string.

Think about how you find and replace words using a pattern.

Describe how backreferences can be used in the replacement string of preg_replace.

Imagine swapping parts of a matched phrase.