There are several options to accomplish this goal.
I'm assuming that the output from {EEG.event.code} is a cell-array of character vectors. So that my code below works, I'm going to hard-code a bunch of codes.
codes = {'notstm','abc','def','stm+','stm-','ghi','nope','jkl','stm-','mno','stm+','stmnot-','stmnot+'};
If I'm understanding correctly, your goal is to find the index of either 'stm+' or 'stm-' (which, in this case, should be 4,5, 9, and 11.
Option 1
index = find(codes == "stm+" | codes == "stm-")
Option 2
index = find(ismember(codes,{'stm+','stm-'}))
Option 3
When you combine two scalar strings using the boolean "or" (|) operator, you get a pattern object that matches either string, then you can call matches to look for valid matches to your pattern.
Note that when you call matches you can specify whether to IgnoreCase or not.
pat = "stm+" | "stm-"
index = find(matches(codes, pat))
Option 4
This is just a variant of the previous example.
pat = "stm" + ("+" | "-")
index = find(matches(codes, pat))
