$1','\\emph{Hello {there} !}'); //Hello {there} ! $Balise=array ( '#\\\\emph{(.+)}#isU'=>'$1', '#\\\\big{(.+)}#isU'=>'$1', '#\\\\acronym\[(.+)\]{(.+)}#isU'=>'$2',//Acronym. Please note interlaced [] aren't treated on this article ); echo preg_replace_wb(array_keys($Balise),array_values($Balise),'The \\emph{\\acronym[Federal Bureau of Investigation]{F.\\big{B}.I.}} killed me.');//. Output : The F.B.I. killed me.