Sanitizing strings to make them URL and filename safe?

Some observations on your solution:

1. 'u' at the end of your pattern means that the pattern, and not the text it's matching will be interpreted as UTF-8 (I presume you assumed the latter?).
2. \w matches the underscore character. You specifically include it for files which leads to the assumption that you don't want them in URLs, but in the code you have URLs will be permitted to include an underscore.
3. The inclusion of "foreign UTF-8" seems to be locale-dependent. It's not clear whether this is the locale of the server or client. From the PHP docs:

A "word" character is any letter or digit or the underscore character, that is, any character which can be part of a Perl "word". The definition of letters and digits is controlled by PCRE's character tables, and may vary if locale-specific matching is taking place. For example, in the "fr" (French) locale, some character codes greater than 128 are used for accented letters, and these are matched by \w.

Creating the slug

You probably shouldn't include accented etc. characters in your post slug since, technically, they should be percent encoded (per URL encoding rules) so you'll have ugly looking URLs.

So, if I were you, after lowercasing, I'd convert any 'special' characters to their equivalent (e.g. é -> e) and replace non [a-z] characters with '-', limiting to runs of a single '-' as you've done. There's an implementation of converting special characters here: https://web.archive.org/web/20130208144021/http://neo22s.com/slug

Sanitization in general

OWASP have a PHP implementation of their Enterprise Security API which among other things includes methods for safe encoding and decoding input and output in your application.

The Encoder interface provides:

canonicalize (string $input, [bool $strict = true])
decodeFromBase64 (string $input)
decodeFromURL (string $input)
encodeForBase64 (string $input, [bool $wrap = false])
encodeForCSS (string $input)
encodeForHTML (string $input)
encodeForHTMLAttribute (string $input)
encodeForJavaScript (string $input)
encodeForOS (Codec $codec, string $input)
encodeForSQL (Codec $codec, string $input)
encodeForURL (string $input)
encodeForVBScript (string $input)
encodeForXML (string $input)
encodeForXMLAttribute (string $input)
encodeForXPath (string $input)

https://github.com/OWASP/PHP-ESAPI https://www.owasp.org/index.php/Category:OWASP_Enterprise_Security_API

I found this larger function in the Chyrp code:

/**
 * Function: sanitize
 * Returns a sanitized string, typically for URLs.
 *
 * Parameters:
 *     $string - The string to sanitize.
 *     $force_lowercase - Force the string to lowercase?
 *     $anal - If set to *true*, will remove all non-alphanumeric characters.
 */
function sanitize($string, $force_lowercase = true, $anal = false) {
    $strip = array("~", "`", "!", "@", "#", "$", "%", "^", "&", "*", "(", ")", "_", "=", "+", "[", "{", "]",
                   "}", "\\", "|", ";", ":", "\"", "'", "‘", "’", "“", "”", "–", "—",
                   "—", "–", ",", "<", ".", ">", "/", "?");
    $clean = trim(str_replace($strip, "", strip_tags($string)));
    $clean = preg_replace('/\s+/', "-", $clean);
    $clean = ($anal) ? preg_replace("/[^a-zA-Z0-9]/", "", $clean) : $clean ;
    return ($force_lowercase) ?
        (function_exists('mb_strtolower')) ?
            mb_strtolower($clean, 'UTF-8') :
            strtolower($clean) :
        $clean;
}

and this one in the wordpress code

/**
 * Sanitizes a filename replacing whitespace with dashes
 *
 * Removes special characters that are illegal in filenames on certain
 * operating systems and special characters requiring special escaping
 * to manipulate at the command line. Replaces spaces and consecutive
 * dashes with a single dash. Trim period, dash and underscore from beginning
 * and end of filename.
 *
 * @since 2.1.0
 *
 * @param string $filename The filename to be sanitized
 * @return string The sanitized filename
 */
function sanitize_file_name( $filename ) {
    $filename_raw = $filename;
    $special_chars = array("?", "[", "]", "/", "\\", "=", "<", ">", ":", ";", ",", "'", "\"", "&", "$", "#", "*", "(", ")", "|", "~", "`", "!", "{", "}");
    $special_chars = apply_filters('sanitize_file_name_chars', $special_chars, $filename_raw);
    $filename = str_replace($special_chars, '', $filename);
    $filename = preg_replace('/[\s-]+/', '-', $filename);
    $filename = trim($filename, '.-_');
    return apply_filters('sanitize_file_name', $filename, $filename_raw);
}

Update Sept 2012

Alix Axel has done some incredible work in this area. His phunction framework includes several great text filters and transformations.

• Unaccent
• Slug
• Filter