I haven't found a way to exclude alphabetic characters from a short text field, and I haven't found a way to exclude a specific 3-letter prefix that would always occur either as the first three characters, or if the user was careless, the first three characters after a space.
If I were able to find both filter components, I would need to find a way to combine them in one filter, or use one filter for one component, and reference the first filter in the second. I saw the latter technique in a Community post.
The following Community post answer didn't work for me, nor did others.
https://community.atlassian.com/t5/Jira-Software-questions/JQL-Does-not-contain-operator-usage/qaq-p/906762#U2657717