FEV 2020
Since Jan Fabry anwser, the Wordpress function changed a little bit, so, the correct snippet to version 5.6 is:
add_filter( 'sanitize_title', 'wpse5029_sanitize_title_with_dashes', 10, 3 );
function wpse5029_sanitize_title_with_dashes($title, $raw_title, $context = 'display') {
$title = strip_tags( $raw_title );
// Preserve escaped octets.
$title = preg_replace( '|%([a-fA-F0-9][a-fA-F0-9])|', '---$1---', $title );
// Remove percent signs that are not part of an octet.
$title = str_replace( '%', '', $title );
// Restore octets.
$title = preg_replace( '|---([a-fA-F0-9][a-fA-F0-9])---|', '%$1', $title );
if ( seems_utf8( $title ) ) {
$title = utf8_uri_encode( utf8_encode($title), 200 );
}
if ( 'save' === $context ) {
// Convert  , &ndash, and &mdash to hyphens.
$title = str_replace( array( '%c2%a0', '%e2%80%93', '%e2%80%94' ), '-', $title );
// Convert  , &ndash, and &mdash HTML entities to hyphens.
$title = str_replace( array( ' ', ' ', '–', '–', '—', '—' ), '-', $title );
// Convert forward slash to hyphen.
$title = str_replace( '/', '-', $title );
// Strip these characters entirely.
$title = str_replace(
array(
// Soft hyphens.
'%c2%ad',
// ¡ and ¿.
'%c2%a1',
'%c2%bf',
// Angle quotes.
'%c2%ab',
'%c2%bb',
'%e2%80%b9',
'%e2%80%ba',
// Curly quotes.
'%e2%80%98',
'%e2%80%99',
'%e2%80%9c',
'%e2%80%9d',
'%e2%80%9a',
'%e2%80%9b',
'%e2%80%9e',
'%e2%80%9f',
// Bullet.
'%e2%80%a2',
// ©, ®, °, &hellip, and &trade.
'%c2%a9',
'%c2%ae',
'%c2%b0',
'%e2%80%a6',
'%e2%84%a2',
// Acute accents.
'%c2%b4',
'%cb%8a',
'%cc%81',
'%cd%81',
// Grave accent, macron, caron.
'%cc%80',
'%cc%84',
'%cc%8c',
),
'',
$title
);
// Convert × to 'x'.
$title = str_replace( '%c3%97', 'x', $title );
}
// Kill entities.
$title = preg_replace( '/&.+?;/', '', $title );
$title = str_replace( '.', '-', $title );
$title = preg_replace( '/[^%a-zA-Z0-9 _-]/', '', $title );
$title = preg_replace( '/\s+/', '-', $title );
$title = preg_replace( '|-+|', '-', $title );
$title = trim( $title, '-' );
return $title;
}
The editor will keep showing the lowercase URL, but it will have saved exactly as you sent, without changing the capitalization.
Obs: it might not be a good idea to use this type of function, since the lowercase pattern is more convenient.