# Square brackets in axis label

Within the Plot[] environment I'd like to label one of the axes as

$$r\,\,[a_0]$$

with square brackets to denote that $$a_0$$ is the unit. How can I achieve this? So far I've tried

Text[Style[ToExpression["r [a_0]", TeXForm, HoldForm]]

but this outputs

$$r\,\,(a_0)$$.

So, the subscript comes out correctly but not the brackets.

Use a StringForm:
Plot[x, {x, 0, 1},
AxesLabel -> {Text[StringForm["r[]", Subscript[a, 0]]], Automatic}]

It is alternatively possible to enter the subscripts directly inside the quoted string (without using StringForm).