In WordPress, if you’re displaying your categories list inline instead of block, odds are you’re going to want to be able to define some HTML to separate the outputted items. Since wp_list_categories doesn’t include a separator argument, you need to do a simple PHP string replace to add one in.
I didn’t see any answers to this question on the WordPress forums so I thought I’d post the code I wrote here.
<?php
$needle = '</a>';
$separator = '<span class="pipe"> | </span>'; // fill in your separator here
$cat_list_args = array(
'echo' => __( 0 ), // sends output to variable
'title_li' => __( '' ), // removes title of list (optional)
'hierarchical' => __( 0 ), // makes subcategories not show as inner list items (optional)
'exclude' => __( 1 ), // excludes uncategorized (optional)
);
$cat_list_arr = wp_list_categories( $cat_list_args );
$cat_list_mod = str_replace($needle,$needle.$separator,$cat_list_arr); // switch to $separator.$needle if you want separator inside a tag
echo $cat_list_mod;
?>On a related note, if you want to hide the separator of your final list item, CSS pseudo-classes makes that super easy.
nav li:last-child .pipe { display: none; }