ctype_alnum

ctype_alnum() is a godsend for quick and easy username/data filtering when used in conjunction with str_replace().

Let's say your usernames have dash(-) and underscore(_) allowable and alphanumeric digits as well.

Instead of a regex you can trade a bit of performance for simplicity:

<?php
$sUser = 'my_username01';
$aValid = array('-', '_');

if(!ctype_alnum(str_replace($aValid, '', $sUser))) {
    echo 'Your username is not properly formatted.';
}
?>

It is also important to note that the behavior of `ctype_alnum` differs according to the operating system. For UNIX-based operating system, if you pass a value that is not a string (or an overloaded object), independently of the value, it will always result in false. However, if we do the same on Windows, using, for example, -1 as literal (a minus and a number greater than 0), we'll have true as result.

<?php var_dump(ctype_alnum(-1));
// UNIX: bool(false)
// Windows: bool(true)

Quicktip: If ctype is not enabled by default on your server, replace ctype_alnum($var) with preg_match('/^[a-zA-Z0-9]+$/', $var).

Be aware that this function will not validate non-english alphanumeric characters. Only ASCII.

<?php
$test = "aeiou";
echo ctype_alnum($test) ? "ok\n" : "err\n";
$test = "áeiou";
echo ctype_alnum($test) ? "ok\n" : "err\n";
$test = "aeiöu";
echo ctype_alnum($test) ? "ok\n" : "err\n";
$test = "ñ";
echo ctype_alnum($test) ? "ok\n" : "err\n";

// output:
// ok
// err
// err
// err
?>

In case you want to validate these types of characters, the best choice is the use of regex, but there's a shortcut so that you will not have to bother typing all of them:

This regex: /^[\p{L}\p{M}\p{Nd}]+$/u
\p{L}  = Any letter, syllable, or ideogram from any Unicode script.
\p{M}  = Any combining mark (necessary for vowels/accents in languages like Arabic or Hindi).
\p{Nd} = Any decimal digit (0-9) from any script, excluding fractions like ½ or Roman numerals.
u = The modifier at the end says: treat the string as a UTF-8 representation.

<?php
$test = "aeiouáöñçسअ爱";
echo preg_match('/^[\p{L}\p{M}\p{Nd}]+$/u', $test) ? "ok\n" : "err\n";
$test = "aeiouáöñçسअ爱!+½";
echo preg_match('/^[\p{L}\p{M}\p{Nd}]+$/u', $test) ? "ok\n" : "err\n";

// output:
// ok
// err
?>