<?php
$regex = '~
              _\[{2}  # look for an underscore and two open square brackets
              ([^]]+) # capture anything that is not a closing bracket
              \]{2}_  # followed by two closing square brackets and an underscore
          ~x';        # free space mode for this explanation
$string = "_[[2, verb//substantiv//adjektiv]]_";
 
$new_string = preg_replace($regex, "[[$1]]", $string);
# in the match replace [[(capture Group 1)]]
echo $new_string;
?>

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