<?php $regex = <<<'REGEX' ~ <strong\s*> # Match strong open tag ( # Open group 1 (?: # Non-capturing group (?:(?!</?strong\s*>).) # Match anything that's not strong tag (open/close) | # Or (?R) # Repeat the whole pattern )* # Repeat the non-capturing group zero or more times ) # Close group 1 </strong\s*> # Match strong close tag ~xs REGEX; # The x modifier is for the fancy spacing en commenting # The s modifier is to match newlines with dot "." $output = preg_replace_callback($regex, function($m){ return '<strong>' . preg_replace('~</?strong\s*>~', '', $m[1]) . '</strong>'; }, $input); echo $output; // w1 <strong>w2</strong> w3 w4 <strong>w5 w6</strong> w7
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