php preg_match_all simple regex returns empty values

Question

I need to extract a predefined set of hashtags from a blob of text, then extract what number follows right after it if any. Eg. I'd need to extract 30 from "Test string with #other30 hashtag". I assumed preg_match_all would be the right choice.

Some test code:

$hashtag = '#other';
$string  = 'Test string with #other30 hashtag';
$matches = [];
preg_match_all('/' . $hashtag . '\d*/', $string, $matches);
print_r($matches);

Output:

Array
(
    [0] => Array
        (
            [0] => #other30
        )
)

Perfect... Works as expected. Now to extract the number:

$string = $matches[0][0]; // #other30
$matches = [];
preg_match_all('/\d*/', $string, $matches);
print_r($matches);

Output:

Array
(
    [0] => Array
        (
            [0] =>
            [1] =>
            [2] =>
            [3] =>
            [4] =>
            [5] =>
            [6] => 30
            [7] =>
        )
)

What? Looks like it's trying to match every character?

I'm aware of some preg_match_all related answers (one, two), but they all use a parenthesized subpattern. According to documentation - it is optional.

What am I missing? How do I simply get all matches into an array that match such a basic regex like /\d*/ There doesn't seem to be a more appropriate function in php for that.

I never thought I'd be scratching my head with such a basic thing in PHP. Much appreciated.

Community · Accepted Answer · 2020-06-20 09:12:55Z

2

You need to replace:

preg_match_all('/\d*/', $string, $matches);

with:

preg_match_all('/\d+/', $string, $matches);

Replace * with +

Because

* Match zero or more times.

+ Match one or more times.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Nov 5, 2015 at 10:47

Muhammad Bilal

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1 Comment

Vigintas Labakojis Over a year ago

Such a silly mistake... * matches zero or more of the preceding element... It tries to match zero digits. Duh

anubhava · Accepted Answer · 2015-11-05 10:45:15Z

1

You can use a capturing group:

preg_match_all('/' . $hashtag . '(\d*)/', $string, $matches); 
echo $matches[1][0] . "\n";
//=> 30

Here (\d*) will capture the number after $hashtag.

answered Nov 5, 2015 at 10:45

anubhava

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1 Comment

Vigintas Labakojis Over a year ago

Even better! Thank you!

Community · Accepted Answer · 2017-05-23 10:27:17Z

1

Also see, that you can reset after a certain point to get part of a match by using \K. And of course need to use \d+ instead of \d* to match one or more digits. Else there would be matches in gaps in between the characters where zero or more digits matches.

So your code can be reduced to

$hashtag = '#other';
$string  = 'Test string with #other30 #other31 hashtag';
preg_match_all('/' . $hashtag . '\K\d+/', $string, $matches);
print_r($matches[0]);

See the demo at eval.in and consider using preg_quote for $hashtag.

edited May 23, 2017 at 10:27

CommunityBot

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answered Nov 5, 2015 at 11:19

bobble bubble

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1 Comment

Vigintas Labakojis Over a year ago

Interesting, I wasn't aware of the \K flag. Thank you!

Mi-Creativity · Accepted Answer · 2015-11-05 10:51:06Z

0

PHP Fiddle

<?php

    $hashtag = '#other';
    $string  = 'Test string with #other30 hashtag';
    $matches = [];
    preg_match_all('/' . $hashtag . '\d*/', $string, $matches);
    $string = preg_match_all('#\d+#', $matches[0][0], $m);
    echo $m[0][0];

?>

answered Nov 5, 2015 at 10:51

Mi-Creativity

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