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  1. <?php
  2. // http://stackoverflow.com/q/33730320/5290909
  3.  
  4. 	$y = 'r';
  5. 	$y = preg_quote($y, '/');
  6. 	$x = 'M';
  7. 	$x = preg_quote($x, '/');
  8. 	$subject = 'rrrrrr rrrrr rrrrrr rrrr rrrr.';
  9.  
  10. 	$regex = "/\G(?:^|(?(?<!^.).)..(?:.{4})*?)\K$y/s";
  11.  
  12. 	$result = preg_replace($regex, $x, $subject);
  13.  
  14. 	echo $result;
  15. 	// => MrrMrr MrrrM rrMrrr rrrM rrMr.
  16.  
  17. ?>
Success #stdin #stdout 0.02s 52432KB
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MrrMrr MrrrM rrMrrr rrrM rrMr.
https://ideone.com/AKEGco
Replace char in pos 1,4,8,12... http://stackoverflow.com/q/33730320/5290909
language:
PHP (php 7.3.5)
created:
9 years ago
visibility:
secret

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