For variables use symvar(str). It will create a cell array of variable names.
Finding "built-in" functions can be performed using cellfun(@exist,cell_array_input)==5.
The 5 denotes built-in low level functions. A value of 2 is likely what you also want and these are the Toolbox functions like "filter2", "interp2" or "smooth3".
Another form to use directly on a string is of the form exist('log2','builtin').
To create the cell_array_input from a string use:
cell_array_input=regexp(str, '[a-zA-Z][a-zA-Z0-9]+','match')
This forces the first letter to be alphanumeric but allows functions like interp2 to be captured
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