$re = '/(?sx)
\bblue\b(?=.*(bleu))
|
\bred\b(?=.*(rouge))/sx';
$str = 'blue cheese, a red nose.
bleu rouge';
$subst = "\1\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