Wednesday, 9 May 2012

String character whitelist


How do I determine if a string contains only allowed characters?

Solution (code snippet):

The following PHP example code will check if a string contains only characters in a white-list. It should be pretty easy to convert this to other languages.

function containsOnly($string, $whiteliststring){
    $char_arr = str_split($string);
    $whitelist_arr = str_split($whiteliststring);
    foreach($char_arr as $char){
        if(in_array($char, $whitelist_arr)==false) return false;
    return true;

Example usage of this would be, for instance, checking for numbers only:

$numbers = "0123456789";
$not_only_numbers = "abcd123";
$only_numbers = "123";

echo containsOnly($not_only_numbers, $numbers)?"yes":"no"; // no
echo containsOnly($only_numbers, $numbers)?"yes":"no";     // yes

Code explained:

The containsOnly function uses str_split to convert strings into arrays containing their characters. The loop checks to see if any of the test string characters is absent from the white-list.

This code can generally be sped up by using an implementation of a set rather than arrays.

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