Replace certain part of a php string -

i have text (in specific case $expression), quite long. want output text same way is, except outputting numbers % bold. spelled 3% , there's space 123 %.

<?php $expression = 'here got number 23 % , on'; $tokens = "([0-9]+)[:space:]([\%])"; $pattern = '/[0-9][0-9] %/';  $keyword = array($pattern); $replacement = array("<b>$keyword</b>"); echo preg_replace($keyword, $replacement, $expression); ?> 

this have i'm not sure i'm doing wrong. outputs error on line $replacement = array("<b>$keyword</b>"); , outputs actual string except replaces number% <b>array</b>.

try

$expression = 'here got number 23 % , on'; var_dump(preg_replace('/(\d+\s*\%)/', "<b>$1</b>", $expression));