filters.py

import re

def in_active_path(nav_slug, page_slug):
    return re.match(nav_slug, page_slug)

gistfile1.mustache

{% block primary %}
  <ul>
    {% for slug, title, children in NAVIGATION %}
      {% if slug|in_active_path(page.slug) %}
        <li class="active">
      {% else %}
        <li>
      {% endif %}
          <a href="{{SITEURL}}/{{slug}}/">{{title}}</a>
        </li>
    {% endfor %}
  </ul>
{% endblock %}

{% block secondary %}
<ul id="nav-secondary" class="two columns">
{%- for slug, title, children in NAVIGATION recursive %}
    {% set active = slug|in_active_path(page.slug) %}
    {% if active %}
      <li class="active">
    {% else %}
      <li>
    {% endif %}
      <a href="{{SITEURL}}/{{slug}}/">{{ title }}</a>
      {%- if children and active -%}
        <ul>{{ loop(children) }}</ul>
      {%- endif %}
      </li>
{%- endfor %}
</ul>
{% endblock %}

pelicanconf.py

import os
import sys 
from pelican.plugins import assets

sys.path.append(os.path.dirname(__file__))

from filters import in_active_path

JINJA_FILTERS = { 
    'in_active_path': in_active_path
}