How to add dot(“.”) in post slug

WordPress runs slugs through its sanitize_title_with_dashes() filter function which replaces dots with dashes. Unfortunately the function doesn’t give you any control over that or any ability to change what characters are stripped or replaced.

What we can do however is remove that filter and add our own version of it with a couple of modifications:

remove_filter( 'sanitize_title', 'sanitize_title_with_dashes', 10 );
add_filter( 'sanitize_title', 'wpse231448_sanitize_title_with_dashes', 10, 3 );

function wpse231448_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, 200);
    }

    $title = strtolower($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 );

        // 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

    // WPSE-231448: Commented out this line below to stop dots being replaced by dashes.
    //$title = str_replace('.', '-', $title);

    // WPSE-231448: Add the dot to the list of characters NOT to be stripped.
    $title = preg_replace('/[^%a-z0-9 _\-\.]/', '', $title);
    $title = preg_replace('/\s+/', '-', $title);
    $title = preg_replace('|-+|', '-', $title);
    $title = trim($title, '-');

    return $title;
}

The lines I edited are commented with a “WPSE-231448” – First I commented out the line which does a str_replace() and replaces dots with dashes, then I added the dot to the list of characters to NOT be replaced in the preg_replace() function below that.

Please note that I have not tested this with pagination or anything like that, it simply stops dots being stripped from slugs on the front/backend and any issues that may arise from that will need to be handled accordingly.