function mirror(...) 	--it should be a full program, but then I can't test it many times
						--the ... in full program is the same as here

s=...for i=#s,1,-1 do for j=1,#s-i do m=j+i/2-i%2/2t=s:sub(j,m).."|"..s:sub(m+1,i+j)if t==t.reverse(t)then print(t)return end end end
end

mirror("Hello World")     --> "l|l"
mirror("Programming Puzzles and Code-Golf")     --> Either "m|m" or "z|z"
mirror("abcba")           --> ""
mirror("Hulluh")          --> "ul|lu"
mirror("abcdefggfedcba")  --> "abcdefg|gfedcba"
mirror("abcdefggfabc")    --> "fg|gf"
mirror("AbbA")            --> "Ab|bA"
mirror("This input is a lot like the last one, but with more characters that don't change the output. AbbA") --> "Ab|bA"