This is not very readable by all, but I will try to parse this for you.
1) The command actually being executed is regexp(str, pattern), but it is doing this by knowing that MATLAB takes text after a function as a string. So you could rewrite this by saying:
2) regexp takes the (?@cmd) with cmd being MATLAB code. You can see this in the doc here. regexp doc
So they are really just running the code: