CODEWITHSUNDEEP
phpregex
Guillermo Phillips
Guillermo Phillips

Reputation: 2216

RegEx Named Capturing Groups in PHP

I have the following regex to capture a list of numbers (it will be more complex than this eventually):

$list = '10,9,8,7,6,5,4,3,2,1';

$regex = 
<<<REGEX
    /(?x)
    (?(DEFINE)
        (?<number> (\d+) )
        (?<list> (?&number)(,(?&number))* )
    )
    ^(?&list)/
REGEX;

$matches = array();
if (preg_match($regex,$list,$matches)==1) {
    print_r($matches);
}

Which outputs:

Array ( [0] => 10,9,8,7,6,5,4,3,2,1 )

How do I capture the individual numbers in the list in the $matches array? I don't seem to be able to do it, despite putting a capturing group around the digits (\d+).

EDIT

Just to make it clearer, I want to eventually use recursion, so explode is not ideal:

$match = 
<<<REGEX
    /(?x)
    (?(DEFINE)
        (?<number> (\d+) )
        (?<member> (?&number)|(?&list) )
        (?<list> \( ((?&number)|(?&member))(,(?&member))* \) ) 
    )
    ^(?&list)/
REGEX;

Upvotes: 2

Views: 2997

Answers (3)

Casimir et Hippolyte
Casimir et Hippolyte

Reputation: 89547

The purpose of a (?(DEFINE)...) section is only to define named sub-patterns you can use later in the define section itself or in the main pattern. Since these sub-patterns are not defined in the main pattern they don't capture anything, and a reference (?&number) is only a kind of alias for the sub-pattern \d+ and doesn't capture anything too.

Example with the string: 1abcde2

If I use this pattern: /^(?<num>\d).....(?&num)$/ only 1 is captured in the group num, (?&num) doesn't capture anything, it's only an alias for \d.
/^(?<num>\d).....\d$/ produces exactly the same result.

An other point to clarify. With PCRE (the PHP regex engine), a capture group (named or not) can only store one value, even if you repeat it.

The main problem of your approach is that you are trying to do two things at the same time:

  1. you want to check the format of the string.
  2. you want to extract an unknown number of items.

Doing this is only possible in particular situations, but impossible in general.

For example, with a flat list like: $list = '10,9,8,7,6,5,4,3,2,1'; where there are no nested elements, you can use a function like preg_match_all to reuse the same pattern several times in this way:

if (preg_match_all('~\G(\d+)(,|$)~', $list, $matches) && !end($matches[2])) {
    // \G ensures that results are contiguous
    // you have all the items in $matches[1] 
    // if the last item of $matches[2] is empty, this means
    // that the end of the string is reached and the string
    // format is correct
    echo '<°)))))))>';
}

Now if you have a nested list like $list = '10,9,(8,(7,6),5),4,(3,2),1'; and you want for example to check the format and to produce a tree structure like:

[ 10, 9, [ 8, [ 7, 6 ], 5 ], 4 , [ 3, 2 ], 1 ]

You can't do it with a single pass. You need one pattern to check the whole string format and an other pattern to extract elements (and a recursive function to use it).

<<<FORGET_THIS_IMMEDIATELY

As an aside you can do it with eval and strtr, but it's a very dirty and dangerous way:

eval('$result=[' . strtr($list, '()', '[]') . '];');

FORGET_THIS_IMMEDIATELY;

Upvotes: 2

t.h3ads
t.h3ads

Reputation: 1878

For this simple list, this would be enough (if you have to use a regular expression):

$string = '10,9,8,7,6,5,4,3,2,1';
$pattern = '/([\d]+),?/';

preg_match_all($pattern, $string, $matches);

print_r($matches[1]);

Upvotes: 0

Evadecaptcha
Evadecaptcha

Reputation: 1441

If you mean to get an array of the comma delimited numbers, then explode:

$numbers = explode(',', $matches[0]); //first parameter is your delimiter what the string will be split up by. And the second parameter is the initial string
print_r($numbers);

output:

Array(
    [0] => 10,
    [1] => 9,
    [2] => 8,

etc

Upvotes: 1

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