Try to add these lines to your theme's functions.php
file:
// First of all lets remove standard hook
remove_filter('sanitize_title', 'sanitize_title_with_dashes');
// Add our custom hook for sanitize_title method
add_filter('sanitize_title', 'custom_sanitize_title_with_dashes', 10, 3);
/**
* Sanitizes a title, replacing whitespace and a few other characters with dashes.
*
* Limits the output to alphanumeric characters, underscore (_) and dash (-).
* Whitespace becomes a dash.
*
* @since 1.2.0
*
* @param string $title The title to be sanitized.
* @param string $raw_title Optional. Not used.
* @param string $context Optional. The operation for which the string is sanitized.
* @return string The sanitized title.
*/
function custom_sanitize_title_with_dashes($title, $raw_title = '', $context = 'display') {
$title = strip_tags($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)) {
if (function_exists('mb_strtolower')) {
$title = mb_strtolower($title, 'UTF-8');
}
$title = utf8_uri_encode($title, 3000);
}
$title = strtolower($title);
if ( 'save' == $context ) {
// Convert nbsp, ndash and mdash to hyphens
$title = str_replace( array( '%c2%a0', '%e2%80%93', '%e2%80%94' ), '-', $title );
// Convert nbsp, 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(
// iexcl and iquest
'%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',
// copy, reg, deg, 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 times to x
$title = str_replace( '%c3%97', 'x', $title );
}
$title = preg_replace('/&.+?;/', '', $title); // kill entities
$title = str_replace('.', '-', $title);
$title = preg_replace('/[^%a-z0-9 _-]/', '', $title);
$title = preg_replace('/\s+/', '-', $title);
$title = preg_replace('|-+|', '-', $title);
$title = trim($title, '-');
return $title;
}
Or try this plugin: Longer Permalinks.