("\\(\(.*\)\\)" 1 font-lock-constant-face t)
But the regexp is wrong as the enclosing parentheses are
including in the colouring to. What regexp will let me
choose the text between the parenthesis but not the
parentheses themselves?
("(\\(.*\\))" 1 font-lock-constant-face t)
But you probably don't want to allow parens between the parens, so
maybe
something like this:
("(\\([^()]*\\))" 1 font-lock-constant-face t)
Or if you just want word and symbol constituents between the parens,
then
something like this:
("(\\(\\(\\sw\\|\\s_\\)*\\))" 1 font-lock-constant-face t)