您可以通过替换特殊字符来轻松实现：

jQuery( document ).ready( function( $ ) {
    $( '#discount-products-search' ).on( 'keyup blur', function() {
        let elSearch = $( this );
        let elSearchVal = elSearch.val().toLowerCase();
        
        $( '.discount-products-list .product-item' ).each( function() {
            let _el = $( this );
            let _elA = _el.find( 'a' );
            let _elAText = _elA.text();
            let _elATextLowercase = _elAText.toLowerCase();
            
            if( _elATextLowercase.indexOf( elSearchVal ) > -1 ) {
                _el.show();
                
                _elAText = _elAText.replace( new RegExp( elSearchVal.replace(/[-\/\\^$*+?.()|[\]{}]/g, '\\$&'), 'ig' ), ( match, $1 ) => {
                    return '<mark>' + match + '</mark>';
                } );
                
                _elA.html( _elAText );
            }
            else {
                _el.hide();
                
                _elA.html( _elAText );
            }
        } );
    } );
} );