register private name

Stream: metaprogramming / tactics

[Huajian Xin]

Is there any method to register private name? I tried to run tactic if form of exact match ... with | ... | ... and got the error message failed to register private name '_match', , prefix has not been registered. Is there any way to figure out that?

[Alex J. Best]

Can you give a full #mwe of what you are trying to do?