Function isOperator

Returns op is
    | "match"
    | "is"
    | "="
    | "=="
    | "!="
    | "<>"
    | ">"
    | ">="
    | "<"
    | "<="
    | "in"
    | "not in"
    | "is not"
    | "like"
    | "not like"
    | "ilike"
    | "not ilike"
    | "@>"
    | "<@"
    | "^@"
    | "&&"
    | "?"
    | "?&"
    | "?|"
    | "!<"
    | "!>"
    | "<=>"
    | "!~"
    | "~"
    | "~*"
    | "!~*"
    | "@@"
    | "@@@"
    | "!!"
    | "<->"
    | "regexp"
    | "is distinct from"
    | "is not distinct from"
    | "+"
    | "-"
    | "*"
    | "/"
    | "%"
    | "^"
    | "&"
    | "|"
    | "#"
    | "<<"
    | ">>"
    | "->"
    | "->>"
    | "||"
    | "exists"
    | "not exists"
    | "not"
    | "between"
    | "between symmetric"