<?php
 
$string = <<<DATA
 
 
    [SYS 1]Page 1 from 2:[/SYS]
 
    Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard ...
 
    [SYS 2]Page 2 from 2:[/SYS]
 
    It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged.
DATA;
 
$regex = '~\[SYS[^]]*\](.+?)\[/SYS\]~';
$string = preg_replace($regex, '$1', $string);
 
echo $string;
 
?>

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