I've one location match that properly works:

<LocationMatch "^/user/storage/subaccounts(/[a-zA-Z0-9]+)?/?$">

this properly forwards for something similar to /user/storage/subaccounts/exampleuser or /user/storage/subaccounts/

Let me setup a locationmatch for /user/storage/subaccounts/exampleuser/admin

My attempt for it was:

<LocationMatch "^/user/storage/subaccounts(/[a-zA-Z0-9]+)?/?admin/?$">

Which does not work. Would someone be so kind as to indicate my error and assist me to having a correct version? Thanks!

Although there's an needless question mark before "admin", this will work acceptable for the sample URL you allow. The mistake is probably not within the regex however the directive are applying on caused by the regex. <LocationMatch> alone doesn't inflict redirection - it really matches the URL.