How does one use the matching regex?

[UtahKemeran]

I have a snippet defined as follows:

context math(context)
snippet `vec([a-zA-Z])` "bold vector" iA
``rv = \textbf{m[1]}`` $0
endsnippet

How can I use the captured group from the snippet trigger word, e.g. if I type veca it should become \textbf{a}

[UtahKemeran]

\textbf{``rv = m[1]``}