Regex Capturing Groups

Summary: in this tutorial, you will learn how to use the regex capturing groups to group and capture parts of a match.

Introduction to the regex capturing groups #

Suppose you have a URI with the following format:

'posts/25'

The URI has a resource name (posts) and id (25). The resource name is a string, while the resource id is an integer.

To match this URI, you can use the following pattern:

\w+/\d+

The following describes the pattern:

\w+ – start with one or more words
/ – contains the forward slash (/).
\d+ – end with one or more number

Since the pattern contains the forward-slash (/), it’s more readable to use the curly braces as the delimiters to form the regular expression:

"{\w+/\d+}"

The following uses the preg_match() function to match the URI:

<?php

$uri = 'posts/25';
$pattern = '{\w+/\d+}';

if (preg_match($pattern, $uri, $matches)) {
    print_r($matches);
}

Try it

Here’s the output:

Array
(
    [0] => posts/25
)

To get the id from the URI, you can use a capturing group.

A capturing group allows you to get a part of the match as a separate item in the result array.

To create a capturing group, you place part of the pattern in parentheses (...). For example, to capture the id from the URI above, you can use the following regular expression with a capturing group that captures the \d+ part:

'{\w+/(\d+)}'

The following shows the updated code with the capturing group:

<?php

$uri = 'posts/25';
$pattern = '{\w+/(\d+)}';

if (preg_match($pattern, $uri, $matches)) {
    print_r($matches);
}

Try it

Output:

Array
(
    [0] => posts/25
    [1] => 25
)

To $matches array now includes both the match and the capturing group. Also, you can have multiple capturing groups like this:

<?php

$uri = 'posts/25';
$pattern = '{(\w+)/(\d+)}';

if (preg_match($pattern, $uri, $matches)) {
    print_r($matches);
}

Try it

Output:

Array
(
    [0] => posts/25
    [1] => posts
    [2] => 25
)

Regex named groups #

You can put the ?<name> syntax immediately after the opening parenthesis to name a capturing group. For example:

<?php

$uri = 'posts/25';
$pattern = '{(?<controller>\w+)/(?<id>\d+)}';

if (preg_match($pattern, $uri, $matches)) {
    print_r($matches);
}

Try it

Output:

Array
(
    [0] => posts/25
    [controller] => posts
    [1] => posts
    [id] => 25
    [2] => 25
)

In this example, we assign the first part of the URI the name controller and the second part the name id.

To get only controller and id from the $matches array, you can pass the $matches array to the array_filter() function like this:

<?php

$uri = 'posts/25';
$pattern = '{(?<controller>\w+)/(?<id>\d+)}';

if (preg_match($pattern, $uri, $matches)) {
    $parts = array_filter($matches, fn($key) => is_string($key), ARRAY_FILTER_USE_KEY);
    print_r($parts);
}

Try it

Output:

Array
(
    [controller] => posts
    [id] => 25
)

Note that PHP MVC frameworks often use this technique to resolve the URI with a controller and query parameters.

More regex capturing groups example #

Suppose you need to match the following pattern:

controller/year/month/day

And you want to capture the controller, year, month, and day.

To do that, you use the named groups for capturing groups in a pattern like the following:

<?php

// controller/year/month/day
$uri = 'posts/2021/09/12';

$pattern = '{(?<controller>\w+)/(?<year>\d{4})/(?<month>\d{2})/(?<day>\d{2})}';

if (preg_match($pattern, $uri, $matches)) {
    // only get string key
    $parts = array_filter($matches, fn($key) => is_string($key), ARRAY_FILTER_USE_KEY);
    print_r($parts);
}

Try it

Output:

Array
(
    [controller] => posts
    [year] => 2021
    [month] => 09
    [day] => 12
)

Reference regex capturing groups in replacement strings #

Suppose you have the name of a person in the first name and last name order e.g., 'John Doe' and you want to reformat it in the reverse order like 'Doe, John':

$name = 'John Doe'; // turns into 'Doe, John'

To match the name format, you can use the following regular expression:

'{\w+ \w+}'

To capture the first name and last name in the matches array, you can put the \w+ pattern in parentheses:

'{(\w+) (\w+)}'

The preg_replace() function allows you to reference a capturing group by its number using the $n format, where n is the capturing group number.

So in the following pattern:

'{(\w+) (\w+)}'

The $1 references the capturing group for the first name and $2 references the capturing group for the last name.

The following shows how to use the preg_replace() function to swap the first name and last name and place a comma between them:

<?php

$name = 'John Doe';
$pattern = '{(\w+) (\w+)}';

echo preg_replace($pattern, '$2, $1', $name);

Try it

Output:

Doe, John

Summary #

Use a regex capturing group to get a part of the match as a separate item in the result array.
Put a part of the pattern in parentheses (...) to create a capturing group.
Assign a capturing group a name by putting the ?<name> immediately after the opening parentheses (?<name>...).
Use $n to reference a capturing group, where n is the capturing group number.

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