<?php

	$string = 'antes[caption id="blabla"](aqui aparece a imagem)[/caption]depois[b]xx[/b]';

    echo "Teste1\n";
	echo preg_replace('/\[\/?caption.*\]/U', '', $string );

    echo "\n\nTeste2\n";
	echo preg_replace('/\[.*\]/U', ' ', $string );

    echo "\n\nTeste3\n";
    echo preg_replace( '/(.*)\[caption.*\](.*)\[\/caption\](.*)/', '$1 { $2 } $3', $string );
    
    echo "\n\nTeste4\n";
    echo preg_replace( '/.*\[caption.*\](.*)\[\/caption\].*/', '$1', $string );