Capturing groups and backreferences in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a capturing group in a regular expression?

A capturing group is a part of a regular expression enclosed in parentheses (). It saves the matched text so you can use it later in your code or in the same pattern.

beginner

How do you refer to a captured group inside the same regular expression?

You use a backreference, which is written as \1, \2, etc., where the number matches the order of the capturing group.

intermediate

Example: What does the pattern '/(\w+) \1/' match?

It matches two identical words next to each other, like 'hello hello'. The first (\w+) captures a word, and \1 checks if the next word is the same.

intermediate

In PHP, how do you access the text matched by capturing groups after using preg_match?

The matched groups are stored in an array passed by reference to preg_match. The full match is at index 0, and captured groups start from index 1.

beginner

Why are capturing groups useful in programming?

They let you extract parts of a string easily, reuse matched text, and perform complex text manipulations like swapping words or validating repeated patterns.

What symbol is used to create a capturing group in a regular expression?

A ()

B []

C {}

D <>

Parentheses () are used to create capturing groups in regular expressions.

In PHP regex, what does \2 refer to?

A The second word in the string

B The second character in the string

C The second line of input

D The second capturing group

\2 is a backreference to the second capturing group in the regex.

What will '/(\d+)\1/' match in the string '123123'?

A Any digits

B Only one digit

C Two identical sequences of digits in a row

D No match

The pattern matches a sequence of digits followed immediately by the same sequence again.

After preg_match('/(foo)(bar)/', 'foobar', $matches), what is in $matches[1]?

A null

B foo

C foobar

D bar

$matches[1] contains the text matched by the first capturing group, which is 'foo'.

Which of these is NOT a use of capturing groups?

A Changing the case of letters automatically

B Reusing matched text in the same regex

C Extracting parts of a string

D Validating repeated patterns

Capturing groups do not change letter case automatically; they only capture and reference matched text.

Explain what capturing groups and backreferences are and how they work together in regular expressions.

Think about how you can remember and reuse parts of a matched string.

Describe how you would use capturing groups and backreferences in PHP to find repeated words in a sentence.

Focus on matching a word and then the same word again.