PHP replace all instances with single regex pattern

Question

I have a single regex and I want to replace each match in the array of matches with a corresponding array of replacements in the most efficient way possible.

So for instance, I have:

$string = '~~hello~~ there, how ~~are~~ ~~you~~?';

$pattern = '/~~(.*?)~~/';

$replacements = array();
$replacements[0] = 'hi';
$replacements[1] = 'am';
$replacements[2] = 'i';

and I want to turn $string into:

hi there, how am i?

Initially I hoped it'd be as simple as:

$string = preg_replace($pattern, $replacements, $string);

but it doesn't seem to work. So the first question is: if $replacements is an array, then does $string must also be an array?

Now, I can come up with (seemingly) inefficient ways to do this, like counting the number of matches and making an array filled with the appropriate number of identical regexes. But this leads us into question two: is there a more efficient way? How would you do it, PHP pros?

Answer 1

You can use a simple eval trick here:

print preg_replace('/~~(\w+)~~/e', 'array_shift($replacements)', $st);

array_shift will simply fetch the first entry from your replacement array.

Better would be using a map though ("hello" => "hi").

Answer 2

I might use preg_replace_callback:

$string = '~~hello~~ there, how ~~are~~ ~~you~~?';

$pattern = '/~~(.*?)~~/'; 

var_dump(preg_replace_callback($pattern, 
    function($matches) { 
        static $replacements = array('hi', 'am', 'i'), $i = 0; 
        return $replacements[$i++ % count($replacements)]; 
    }, 
    $string));

Output:

string(19) "hi there, how am i?"

Answer 3

If all you're looking to do is switch out those three specific phrases with another set of specific phrases, then you can just use str_replace as it is much faster than preg_replace.

$subject = "~~hello~~ there, how ~~are~~ ~~you~~?";
$matches = array('~~hello~~', '~~are~~', '~~you~~');
$replace = array('hi', 'am', 'i');

str_replace($matches, $replace, $subject);