vector.sty

\ProvidesPackage{vector}
% based on: http://tex.stackexchange.com/questions/167172/consistent-macro-for-bold-upright-vectors-in-both-latin-and-greek

\RequirePackage{xparse}
\RequirePackage{bm,upgreek}

\ExplSyntaxOn
\NewDocumentCommand\vect{m}
{
	\commexo_vector:n { #1 }
}

\cs_new_protected:Npn \commexo_vector:n #1
{
	\tl_map_inline:nn { #1 }
	{
		\commexo_vector_inner:n { ##1 }
	}
}

\cs_new_protected:Npn \commexo_vector_inner:n #1
{
	\tl_if_in:VnTF \g_commexo_latin_tl { #1 }
	{% we check whether the argument is a Latin letter
		\mathbf { #1 } % a Latin letter
	}
	{% if not a Latin letter, we check if it's an uppercase Greek letter
		\tl_if_in:VnTF \g_commexo_ucgreek_tl { #1 }
		{
			\bm { #1 } % a Greek uppercase letter
		}
		{% if not, we check if it's a lowercase Greek letter
			\tl_if_in:VnTF \g_commexo_lcgreek_tl { #1 }
			{
				\commexo_makeboldupright:n { #1 }
			}
			{% none of the above, just issue #1
				#1 % fall back
			}
		}
	}
}

\cs_new_protected:Npn \commexo_makeboldupright:n #1
{
	\bm { \use:c { up \cs_to_str:N #1 } }
}

\tl_new:N \g_commexo_latin_tl
\tl_new:N \g_commexo_ucgreek_tl
\tl_new:N \g_commexo_lcgreek_tl
\tl_gset:Nn \g_commexo_latin_tl
{
	ABCDEFGHIJKLMNOPQRSTUVWXYZ
	abcdefghijklmnopqrstuvwxyz
	0123456789
}
\tl_gset:Nn \g_commexo_ucgreek_tl
{
	\Gamma\Delta\Theta\Lambda\Pi\Sigma\Upsilon\Phi\Chi\Psi\Omega
}
\tl_gset:Nn \g_commexo_lcgreek_tl
{
	\alpha\beta\gamma\delta\epsilon\zeta\eta\theta\iota\kappa
	\lambda\mu\nu\xi\pi\rho\sigma\tau\upsilon\phi\chi\psi\omega
	\varepsilon\vartheta\varpi\varphi\varsigma\varrho
}

\ExplSyntaxOff
\endinput