PHP Get the word replaced by preg_replace - php

Question

How can I get the replaced word by preg_replace() function.
preg_replace('/[#]+([A-Za-z0-9-_]+)/', '$0', $post );
I want to get $1 variable so that I can user it further.

Answer 1

Capture it before you replace the expression:
// This is where the match will be kept
$matches = array();
$pattern = '/[#]+([A-Za-z0-9-_]+)/';
// Check if there are matches and capture the user (first group)
if (preg_match($pattern, $post, $matches)) {
// First match is the user
$user = $matches[1];
// Do the replace
preg_replace($pattern, '$0', $post );
}

Answer 2

This isn't possible with preg_replace() as it returns the finished string/array, but does not preserve the replaced phrases. You can use preg_replace_callback() to manually achieve this.
$pattern = '/[#]+([A-Za-z0-9-_]+)/';
$subject = '#jurgemaister foo #hynner';
$tokens = array();
$result = preg_replace_callback(
$pattern,
function($matches) use(&$tokens) {
$tokens[] = $matches[1];
return ''.$matches[0].'';
},
$subject
);
echo $result;
// #jurgemaister foo #hynner
print_r($tokens);
// Array
// (
// [0] => jurgemaister
// [1] => hynner
// )

Answer 3

You should use preg_match in addition to preg_replace. preg_replace is just for replacing.
$regex = '/[#]+([A-Za-z0-9-_]+)/';
preg_match($regex, $post, $matches);
preg_replace($regex, '$0', $post );

Answer 4

You can't do that with preg_replace, but you can do it with preg_replace_callback:
preg_replace_callback($regex, function($matches){
notify_user($matches[1]);
return "<a href='/$matches[1]' target='_blank'>$matches[0]</a>";
}, $post);
replace notify_user with whatever you would call to notify the user.
This can also be modified to check whether the user exists and replace only valid mentions.