How posted tags are filtered in q2a core? Cannot find the code in core that removes special characters.

I am trying to understand (find in the code) how q2a filters the tags and remove special characters.

When posting e.g. "graph! dogs# test? test2` " it results in a cleaned tagstring:

graph dogs# test test2

But I cannot find the function in the code doing it.

I found only:
- qa_post_tags_to_tagstring($tags)
- qa_tags_to_tagstring()

But these functions are only splitting the tagstring.