$(echo foo)
`echo foo`
"$(echo foo) bar"

----------------------------------------------------

[
	["variable", [
		["variable", "$("],
		["keyword", "echo"], 
		" foo",
		["variable", ")"]
	]],
	["variable", [
		["variable", "`"],
		["keyword", "echo"], 
		" foo",
		["variable", "`"]
	]],
	["string", [
		"\"",
		["variable", [
			["variable", "$("],
			["keyword", "echo"], 
			" foo",
			["variable", ")"]
		]],
		" bar\""
	]]
]

----------------------------------------------------

Checks for command substitution.