Custom template filter: replace HTML element with regex filtered text (Django 2.1)

Question

I am building a search engine, which needs a custom filter that displays the text surrounding a keyword, like the excerpts on Google results page. I am using regex to identify the surrounding words. Here is my code for the filter:

@register.filter(needs_autoescape=True)
@stringfilter
def show_excerpt (value, search_term, autoescape=True):
    # make the keyword put into the search engine case insensitive #
    keywords = re.compile(re.escape(search_term), re.IGNORECASE)
    # make excerpt return 300 characters before and after keyword #
    excerpt_text = '.{300}' + str(keywords) + '.{300}'
    # replace the original text with excerpt #
    excerpt = value.sub(excerpt_text, value)
    return mark_safe(excerpt)

Code for the search engine in view.py:

def query_search(request):
    articles = cross_currents.objects.all()
    search_term = ''
    if 'keyword' in request.GET:
        search_term = request.GET['keyword']
        articles = articles.annotate(similarity=Greatest(TrigramSimilarity('Title', search_term), TrigramSimilarity('Content', search_term))).filter(similarity__gte=0.03).order_by('-similarity')
    context = {'articles': articles, 'search_term': search_term}
    return render(request, 'query_search.html', context)

HTML template (it includes a custom highlight filter that highlights the keyword put into search engine):

<ul>
{% for article in articles %}
<li><a href="{% url 'search:article_detail' article.ArticleID %}">{{ article|highlight:search_term }}</a></li>
<p> {{ article.Content|highlight:search_term|show_excerpt:search_term }} </p>
{% endfor %}
</ul>

Error message: 'SafeText' object has no attribute 'sub'

I think I am doing .sub wrong. I just need the excerpt to replace the entire original text (the text that I am putting a filter on). The original text starts from the beginning of the data but I just want to display the data surrounding the keyword, with my highlight custom filter highlighting the keyword (just like on Google). Any idea?

EDIT: when I do re.sub(excerpt_text, value), I get the error message sub() missing 1 required positional argument: 'string'.