Allowed variable names not sufficiently broad

[mykelk]

Right now it is defined like this:

allowed_variable = \
    '(?:[a-zA-Z_\u00A1-\U0010ffff]|%s)(?:[a-zA-Z_0-9\u00A1-\U0010ffff])*!*'

But this seems to leave off, for example, the ∇Q in

∇Q(s,a,r,s′) = (r + γ*U(s′) - Q(θ,s,a))*model.∇Q(θ,s,a)

Note the difference in highlighting:

[mykelk]

This remains an issue. For allowed variable names see: https://docs.julialang.org/en/v1/manual/variables/#Allowed-Variable-Names