You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: makeValidFieldName.m
    path of file: ./repos/Machine-Learning-homework/machine-learning-ex8/ex8/lib
    
    the code of the file until where you have to start completion: function str = makeValidFieldName(str)
% From MATLAB doc: field names must begin with a letter, which may be
% followed by any combination of letters, digits, and underscores.
% Invalid characters will be converted to underscores, and the prefix

    
function str = makeValidFieldName(str)
% From MATLAB doc: field names must begin with a letter, which may be
% followed by any combination of letters, digits, and underscores.
% Invalid characters will be converted to underscores, and the prefix
% "x0x[Hex code]_" will be added if the first character is not a letter.
    isoct=exist('OCTAVE_VERSION','builtin');
    pos=regexp(str,'^[^A-Za-z]','once');
    if(~isempty(pos))
        if(~isoct)
            str=regexprep(str,'^([^A-Za-z])','x0x${sprintf(''%X'',unicode2native($1))}_','once');
        else
            str=sprintf('x0x%X_%s',char(str(1)),str(2:end