Regex lookaheads (or behinds) matching foo bar or bar foo
/^(?=.*foo)(foo )?bar baz( foo)?$/

/foo bar baz|bar baz foo/

$foo = 'foo'
$barbaz = 'bar baz'
$regex = "($foo $barbaz)|($barbaz $foo)"

(foo bar baz|bar baz foo)

(foo (bar|baz)|(bar|baz) foo)