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<?php $string = "Hello {replace this curley} {{but not this one}} {{{and not this one}}}"; $formatted = preg_replace('/^(?!\\{{2,})(?:[^\\{]*)(\\{{1})(?:[^\\{\\}]*)(\\}{1})$/',"",$string); var_dump($formatted);

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71.01 ms | 402 KiB | 5 Q