$re = '/(0|1|2|3|4|5|6|7|8|9)
(?=.*:\1=(\w+))/sx';
$str = '0 zero
1 ichi
2 ni
3 san
4 shi
5 go
6 roku
7 shichi
8 hachi
9 kyuu
Dictionary:0=0:1=1:2=2:3=3:4=4:5=5:6=6:7=7:8=8:9=9';
$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