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  1. <?php
  2.  
  3. $string = '<br/>    <br>
  4.  
  5. <!--
  6. this   is a comment
  7.  
  8. -->
  9. <br/>   <br/>';
  10.  
  11. $new_string = preg_replace('#(?(?!<!--.*?-->)(?: {2,}|[\r\n\t]+)|(<!--.*?-->))#s', '$1', $string);
  12. echo $new_string;
  13. ?>
Success #stdin #stdout 0.01s 20568KB
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Standard input is empty
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<br/><br><!--
this   is a comment

--><br/><br/>
https://ideone.com/7wt9g5
language:
PHP (php 7.3.5)
created:
11 years ago
visibility:
public

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