Replace regex with Jinja2

You can use the following syntax:

<input mixvalidate="^.{ {{-USER_USERNAME_MIN}},{{USER_USERNAME_MAX}}}$">

The {{ }} wrappers are the standard way to inject jinja variables. Here a space is used to distinguish the literal opening brace from such injection-syntax, and the minus sign is used to remove that white space in the final result.

See it on packetcoders.io/j2-render

If you want to assert that the username is between 2 and 20 characters, then use this regular expression:

^.{2,20}$

Updated code:

<input mixvalidate="^.{2,20}$">

You can make it dynamic with Jinja2 so you don’t have to change the regex in multiple places. For your case:

<input mixvalidate="^.{ {{ USER_USERNAME_MIN }},{{ USER_USERNAME_MAX }} }$">

{{ USER_USERNAME_MIN }} and {{ USER_USERNAME_MAX }} will be replaced with the values from app.py.

Make sure you pass those variables to your template:

from flask import render_template # or FastAPI/Jinja2 equivalent return render_template("template.html", USER_USERNAME_MIN=USER_USERNAME_MIN, USER_USERNAME_MAX=USER_USERNAME_MAX)

✅ Now if you change USER_USERNAME_MIN or USER_USERNAME_MAX in your Python file, the input validation updates automatically.