<?php $arr = [ 'y' => 35, 'x' => 51, 'c_3' => 4, 'c_1' => 54, 'c_6' => 53, 'c_9' => 52, 'z' => 4, ]; var_export( array_filter( $arr, function($v) use($arr) { return preg_match( '#c_\d#', array_search($v, $arr) ); } ) );
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