3 added 92 characters in body
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Octave, 34 bytes

@(s)mod(nnz(~(s'-'aeiouAEIOU')),2)


s'-'aeiouAEIOU'    % Creates a 2D-matrix where each of the odd letters are 
                   % subtracted from the string s
~(s'-'aeiouAEIOU') % Negates that array, so the all zero elements become 1
nnz( .... )        % Counts all the non-zero elements (the odd letters)
mod(nnz( ....),2   % Takes this sum modulus 2

This is 6 bytes shorter than the traditional approach using ismember, @(s)mod(sum(ismember(s,'aeiouAEIOU')),2), and two bytes shorter than the regex approach: @(s)mod(nnz(regexpi(s,'[aeiou]')),2).

Test it here.

Octave, 34 bytes

@(s)mod(nnz(~(s'-'aeiouAEIOU')),2)


s'-'aeiouAEIOU'    % Creates a 2D-matrix where each of the odd letters are 
                   % subtracted from the string s
~(s'-'aeiouAEIOU') % Negates that array, so the all zero elements become 1
nnz( .... )        % Counts all the non-zero elements (the odd letters)
mod(nnz( ....),2   % Takes this sum modulus 2

This is 6 bytes shorter than the traditional approach using ismember.

Test it here.

Octave, 34 bytes

@(s)mod(nnz(~(s'-'aeiouAEIOU')),2)


s'-'aeiouAEIOU'    % Creates a 2D-matrix where each of the odd letters are 
                   % subtracted from the string s
~(s'-'aeiouAEIOU') % Negates that array, so the all zero elements become 1
nnz( .... )        % Counts all the non-zero elements (the odd letters)
mod(nnz( ....),2   % Takes this sum modulus 2

This is 6 bytes shorter than the traditional approach using ismember, @(s)mod(sum(ismember(s,'aeiouAEIOU')),2), and two bytes shorter than the regex approach: @(s)mod(nnz(regexpi(s,'[aeiou]')),2).

Test it here.

2 added 388 characters in body
source | link

Octave, 34 bytes

@(s)mod(nnz(~(s'-'aeiouAEIOU')),2) 


s'-'aeiouAEIOU'    % Creates a 2D-matrix where each of the odd letters are 
                   % subtracted from the string s
~(s'-'aeiouAEIOU') % Negates that array, so the all zero elements become 1
nnz( .... )        % Counts all the non-zero elements (the odd letters)
mod(nnz( ....),2   % Takes this sum modulus 2

This is 6 bytes shorter than the traditional approach using ismember.

Test it here.

Octave, 34 bytes

@(s)mod(nnz(~(s'-'aeiouAEIOU')),2)

Octave, 34 bytes

@(s)mod(nnz(~(s'-'aeiouAEIOU')),2) 


s'-'aeiouAEIOU'    % Creates a 2D-matrix where each of the odd letters are 
                   % subtracted from the string s
~(s'-'aeiouAEIOU') % Negates that array, so the all zero elements become 1
nnz( .... )        % Counts all the non-zero elements (the odd letters)
mod(nnz( ....),2   % Takes this sum modulus 2

This is 6 bytes shorter than the traditional approach using ismember.

Test it here.

1
source | link

Octave, 34 bytes

@(s)mod(nnz(~(s'-'aeiouAEIOU')),2)