Simultaneous Preg_replace operation in php and regex

Question

I know many of the users have asked this type of question but I am stuck in an odd situation.

I am trying a logic where multiple occurance of a specific pattern having unique identifier will be replaced with some conditional database content if there match is found.

My regex pattern is

'/{code#(\d+)}/'

where the 'd+' will be my unique identifier of the above mentioned pattern.

My Php code is:

This works but it is not at all dynamic, the numbers after #tag from pattern are ids i.e 1,2 & 3 and their respective data is stored in database.

how could I access the content from DB of respective ID mentioned in the pattern and would replace the entire pattern with respective content.

I am really not getting a way of it. Thank you in advance

HamZa · Accepted Answer

It's not that difficult if you think about it. I'll be using PDO with prepared statements. So let's set it up:

$db = new PDO(                                      // New PDO object
    'mysql:host=localhost;dbname=projectn;charset=utf8', // Important: utf8 all the way through
    'username',
    'password',
    array(
        PDO::ATTR_EMULATE_PREPARES => false,        // Turn off prepare emulation
        PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION
    )
);

This is the most basic setup for our DB. Check out this thread to learn more about emulated prepared statements and this external link to get started with PDO.

We got our input from somewhere, for the sake of simplicity we'll define it:

$text = 'The old version is {code#1}, The new version is {code#2}, The stable version {code#3}';

Now there are several ways to achieve our goal. I'll show you two:

1. Using preg_replace_callback():

$output = preg_replace_callback('/{code#(\d+)}/', function($m) use($db) {
    $stmt = $db->prepare('SELECT `content` FROM `footable` WHERE `id`=?');
    $stmt->execute(array($m[1]));
    $row = $stmt->fetch(PDO::FETCH_ASSOC);
    if($row === false){
        return $m[0]; // Default value is the code we captured if there's no match in de DB
    }else{
        return $row['content'];
    }
}, $text);

echo $output;

Note how we use use() to get $db inside the scope of the anonymous function. ^{^{global is evil}}

Now the downside is that this code is going to query the database for every single code it encounters to replace it. The advantage would be setting a default value in case there's no match in the database. If you don't have that many codes to replace, I would go for this solution.

2. Using preg_match_all():

if(preg_match_all('/{code#(\d+)}/', $text, $m)){
    $codes = $m[1];     // For sanity/tracking purposes
    $inQuery = implode(',', array_fill(0, count($codes), '?')); // Nice common trick: https://stackoverflow.com/a/10722827
    $stmt = $db->prepare('SELECT `content` FROM `footable` WHERE `id` IN(' . $inQuery . ')');
    $stmt->execute($codes);
    $rows = $stmt->fetchAll(PDO::FETCH_ASSOC);

    $contents = array_map(function($v){
        return $v['content'];
    }, $rows); // Get the content in a nice (numbered) array

    $patterns = array_fill(0, count($codes), '/{code#(\d+)}/'); // Create an array of the same pattern N times (N = the amount of codes we have)
    $text = preg_replace($patterns, $contents, $text, 1); // Do not forget to limit a replace to 1 (for each code)
    echo $text;
}else{
    echo 'no match';
}

The problem with the code above is that it replaces the code with an empty value if there's no match in the database. This could also shift up the values and thus could result in a shifted replacement. Example (code#2 doesn't exist in db):

Input: foo {code#1}, bar {code#2}, baz {code#3}
Output: foo AAA, bar CCC, baz
Expected output: foo AAA, bar , baz CCC

The preg_replace_callback() works as expected. Maybe you could think of a hybrid solution. I'll let that as a homework for you :)

Simultaneous Preg_replace operation in php and regex

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