It is possible to obtain the canonical basis from the PBW basis, as long as you are working in the quantum group U<sub>q</sub>(n). The canonical basis seems to be inherently a quantum phenomenon, so it shouldn't be surprising that we want to compute in the quantum group.

If E<sub>&pi;</sub> is an indexing of the quantum PBW basis, then what one first proves is that applying the bar involution is unitriangular with respect to this basis. There is even a mathoverflow question about this: https://mathoverflow.net/questions/110108/.

Now it is a matter of linear algebra to prove that there exists a unique bar invariant basis b<sub>&pi;</sub> such that
$$b_\pi=\sum_\sigma c_{\pi\sigma} E_\sigma$$
with c<sub>&sigma;&sigma;</sub>=1, c<sub>&sigma;&pi;</sub>&in;qℤ[q] if &sigma;&#x2260;&pi; and c<sub>&sigma;&pi;</sub>=0 unless &sigma;&leq;&pi;. This basis b<sub>&pi;</sub> is the canonical basis.