PHP function to Remove Non AlphaNumeric Characters from a given string with exclusion list

PHP provides various pattern matching regular expression functions as well as string functions to perform this task.

PHP built-in functions for this are str_replace, str_ireplace, preg_replace, substr_replace ,preg_match

It is recommended to use PHP string replace functions over regex functions to do simple string replace functions.There is nothing wrong with using a regex , regular expressions have to be compiled and strings to be parsed, but some people wrongly them by doing silly things like :
preg_match('/txt/', $str) instead of strpos('txt', $str) !== false      OR
preg_replace("/txt/", "", $str) instead of str_replace('txt', $str)



Using Regular expression PREG_REPLACE Function

PREG_REPLACE is the php built in function to check for a particular string pattern and replace with another one. It performs a regular expression search and replace.

function removeNonAlphanumericChars($str){
return preg_replace("/[^a-zA-Z0-9]/", "", $str);
}

Usage:

$str="%#&&&^&jsdfj%*&%*_^^.pp09==-is9";
echo( removeNonAlphanumericChars($str) );

Or simply

$str="%#&&&^&jsdfj%*&%*_^^.pp09==-is9";
echo( preg_replace("/[^a-zA-Z0-9]/", "", $str));

the "0-9" in the pattern can be replaced with "\d" and the function looks like:

$str="%#&&&^&jsdfj%*&%*_^^.pp09==-is9";
$new_str = preg_replace("/[^a-zA-Z\d]/", "", $str);

Exlusion of Characters from replacement

PHP allows exclusion of characters, if you want to replace all non AlphaNumeric character except dot (.), Hyphen (-) and underscore (_) you can do it like this:

$str="%#&&&^&jsdfj%*&%*_^^.pp09==-is9";
$new_str = preg_replace("/[^a-zA-Z0-9\.\-_]/", "", $str);

The above function will remove all characters which are not small letter alphabets (a-z), capital letter alphabets (A-Z), dot (.), Hyphen (-) and underscore (_).

You can find a backslash (\) before dot and Hyphen. This is because "." and "-" are used as Quantifiers while writing regular expression patterns. So while adding a backslash before those characters will treat them as a string.



Alternative Non Alphanumeric Character function using simple PHP in_array and str_replace functions

Simple PHP function with our reular expression to remove all Non Alphanumeric Characters ina string:

Here there is an array of allowed character "$allowedCharacters". function will loop through the characters in the string and check the presence of character in the array. The characters which are not in the array are replace by "@" character. Later "@" is replace by "" and you will get a Non Alphanumeric Character replaced string.

function removeNonAlphanumericChars($str){

$allowedCharacters = array (
'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z','0','1','2','3','4','5','6','7','8','9'
);

for($i = 0; $i < strlen($str); $i++) {
if (! in_array(strtolower($str[$i]), $allowedCharacters)) {
$str[$i] = '@';
}
}
return str_replace('@', '', $str);
}


$str="%#&&&^&jsdfj%*&%*_^^.pp09==-is9";
echo( removeNonAlphanumericChars($str) );

You can also add exclusions in $allowedCharacters array. Function looks like this:

function removeNonAlphanumericCharsWithExclusions($str){

$allowedCharacters = array (
'.', '-', '_', 'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z','0','1','2','3','4','5','6','7','8','9'
);

for($i = 0; $i < strlen($str); $i++) {
if (! in_array(strtolower($str[$i]), $allowedCharacters)) {
$str[$i] = '@';
}
}
return str_replace('@', '', $str);
}

$str="%#&&&^&jsdfj%*&%*_^^.pp09==-is9";
echo( removeNonAlphanumericCharsWithExclusions($str) );

Hope this helps :)