Problem 695. Parse string and identify specific string sequence in algebraic equation

Given a string S that defines an algebraic expression such as:

S= 'X= A1 + A2*(Y1 + A3*Y3)*exp( A4*Y12 + Y1) ;'

return a cell array {'Y1', 'Y12', 'Y123'}

i.e. parse the string S and identify the unique variables in the expression that start with the letter "Y".