$re = '/(?sx)
(?|
０(?=.*(0))
|１(?=.*(1))
|２(?=.*(2))
|３(?=.*(3))
|４(?=.*(4))
|５(?=.*(5))
|６(?=.*(6))
|７(?=.*(7))
|８(?=.*(8))
|９(?=.*(9))
)/sx';
$str = '０ zero
１ ichi
２ ni
３ san
４ shi
５ go
６ roku
７ shichi
８ hachi
９ kyuu
0123456789';
$subst = "\1";
$result = preg_replace($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