$re = '/(?s)\b(\w+)\b(?=.*:\1=(\w+)\b)/s';
$str = 'blue cheese, a red nose.
Dictionary:blue=bleu:red=rouge:green=vert';
$subst = "\2";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php