Vimgrep / regex question

Not being an expert at regex I have been a bit baffled by how to do a
grep/vimgrep to match. I am probably stepping on my own foot as I
think I have done such in the past but have not been readily able to
do this. Can anyone point me in the right direction?

I am trying to find all lines which begin with 'has_text?' but does
not contain the predicate '.should eq(true)', so this line should get
matched and returned in the grep results:

has_text?('some text','td')

But not this one:

has_text?('some text','td').should eq(true)

I have tried a number of things such as the following but having
trouble getting the 'does not match' predicate to work:

vimgrep /\(has_text?\).*^\(\.should\)/

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