Write2[file, val1 = expr1, val2 = expr2, ...] writes the settings val1 = expr1, val2 = expr2 in sequence followed by a newline, to the specified output file. Setting the option FormatType of Write2 to FortranForm results in Fortran syntax output.
Overview, Isolate, PaVeReduce.
FullForm[$FortranContinuationCharacter]\&
t = Collect[((a - c)^2 + (a - b)^2)^2, a, Factor]4 a^4-8 a^3 (b+c)+8 a^2 \left(b^2+b c+c^2\right)-4 a (b+c) \left(b^2+c^2\right)+\left(b^2+c^2\right)^2
This writes the assignment r=t to a file.
tempfilename = ToString[$SessionID] <> ".s";
Write2[tempfilename, r = t];This shows the contents of the file.
TableForm[ReadList[If[$OperatingSystem === "MacOS", ":", ""] <> tempfilename, String]]\begin{array}{l} \;\text{r = ( 4*a${}^{\wedge}$4 - 8*a${}^{\wedge}$3*(b + c) - 4*a*(b + c)*(b${}^{\wedge}$2 + c${}^{\wedge}$2) + } \\ \;\text{ (b${}^{\wedge}$2 + c${}^{\wedge}$2)${}^{\wedge}$2 + 8*a${}^{\wedge}$2*(b${}^{\wedge}$2 + b*c + c${}^{\wedge}$2)} \\ \;\text{ );} \\ \end{array}
DeleteFile[If[$OperatingSystem === "MacOS", ":", ""] <> tempfilename]t2 = x + Isolate[t, a, IsolateNames -> w]4 a^4-8 a^3 w(19)+8 a^2 w(21)-4 a w(19) w(20)+w(20)^2+x
Write2[tempfilename, r = t2];TableForm[ReadList[If[$OperatingSystem === "MacOS", ":", ""] <> tempfilename, String]]\begin{array}{l} \;\text{w[19] = (b + c} \\ \;\text{ );} \\ \;\text{w[20] = (b${}^{\wedge}$2 + c${}^{\wedge}$2} \\ \;\text{ );} \\ \;\text{w[21] = (b${}^{\wedge}$2 + b*c + c${}^{\wedge}$2} \\ \;\text{ );} \\ \;\text{r = ( 4*a${}^{\wedge}$4 + x - 8*a${}^{\wedge}$3*HoldForm[w[19]] - 4*a*HoldForm[w[19]]*} \\ \;\text{ HoldForm[w[20]] + HoldForm[w[20]]${}^{\wedge}$2 + 8*a${}^{\wedge}$2*HoldForm[w[21]]} \\ \;\text{ );} \\ \end{array}
DeleteFile[If[$OperatingSystem === "MacOS", ":", ""] <> tempfilename]This is how to write out the expression t2 in Fortran format.
Write2[tempfilename, r = t2, FormatType -> FortranForm];TableForm[ReadList[If[$OperatingSystem === "MacOS", ":", ""] <> tempfilename, String]]\begin{array}{l} \;\text{ w(19)= b + c} \\ \;\text{ w(20)= b**2 + c**2} \\ \;\text{ w(21)= b**2 + b*c + c**2} \\ \;\text{ r = x + a**4*4D0 - a**3*8D0*w(19) - a*4D0*w(19)*w(20) + } \\ \;\text{ $\&$ w(20)**2 + a**2*8D0*w(21)} \\ \;\text{ } \\ \end{array}
DeleteFile[If[$OperatingSystem === "MacOS", ":", ""] <> tempfilename];
Clear[w, t, t2, r, tempfilename];