Validate a specific shortcode placeholder string

How to check below line in regular expression?

[albums album_id='41']

All are static except my album_id. This may be 41 or else.

Below my code I have tried but that one not working:

$str = "[albums album_id='41']";
$regex = '/^[albums album_id=\'[0-9]\']$/';
if (preg_match($regex, $str)) {
    echo $str . " is a valid album ID.";
} else {
    echo $str . " is an invalid ablum ID. Please try again.";
}

You need to escape the first [ and add + quantifier to [0-9]. The first [ being unescaped created a character class - [albums album_id=\'[0-9] and that is something you did not expect.

Use

$regex = '/^\[albums album_id=\'[0-9]+\']$/';

Pattern details:

• ^ - start of string
• \[ - a literal [
• albums album_id=\'- a literal string albums album_id='
• [0-9]+ - one or more digits (thanks to the + quantifier, if there can be no digits here, use * quantifier)
• \'] - a literal string ']
• $ - end of string.

See PHP demo:

$str = "[albums album_id='41']";
$regex = '/^\[albums album_id=\'[0-9]+\']$/';
if (preg_match($regex, $str)) {
    echo $str . " is a valid album ID.";
} else {
    echo $str . " is an invalid ablum ID. Please try again.";
}
// => [albums album_id='41'] is a valid album ID.