Find digits in a string jinja

I have a string "abc1234" and want to store the numbers alone as a separate string.

Tried below code. It seems 'i' is passed as string and if condition fails.

{% set a = "12lat12" %}
{% set b = [] %}
{% for i in a %}
{% if i is number %}
{% do b.append(i) %}
{% endif %}
{% endfor %}
{{b|join()}}

If I change the if condition as "if i is string" all characters are printed as such "12lat12".