Here's a simple plugin, it replaces certain text with other text in questions, answers, and comments.
EDIT: now supports regex and html tag substitution
Mostly for simple shortcode replacing, e.g.:
<a href="http://mysite.com/">My Site!</a>
Feedback welcome, the code is here:
Welcome to the Q&A site for Question2Answer.
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April 20, 2021: Q2A 1.8.6 released!