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  1. <?php
  2.  
  3. $arr = [
  4.  'A' => ['A1' => '', 'A2' => ''],
  5.  'B' => ['B1' => ['B1a' => '']]
  6. ];
  7.  
  8. print_r($arr);
  9.  
  10. preg_match_all('/^[^\[]*\[([^\]]*)\][^\]]*$/m', print_r($arr, true), $matches);
  11. $result =  implode(', ', $matches[1]);
  12.  
  13. echo 'Result: ', $result;
Success #stdin #stdout 0.01s 20568KB
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stdin
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Standard input is empty
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Array
(
    [A] => Array
        (
            [A1] => 
            [A2] => 
        )

    [B] => Array
        (
            [B1] => Array
                (
                    [B1a] => 
                )

        )

)
Result: A, A1, A2, B, B1, B1a
https://ideone.com/wGNBET
language:
PHP (php 7.3.5)
created:
12 years ago
visibility:
public

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