{if segment_2}
    {exp:switchee variable="{segment_2}" parse="inward"}
        {case value="#^P(\d+)$#|''"}
            {embed="gallery/listing"}
        {/case}
        {case default="Yes"}
            {embed="gallery/detail"}
        {/case}     
    {/exp:switchee}
{if:else}
    {embed="gallery/listing"}
{/if}