With the demise of the unary operator, + in Google search, I went looking for a reliable list of query operators. (In a unary operation, in a mathematical system, one element is used to yield a single result.)
Google query operators must be entered in lower case. The best list I found is at Search Engine Land. The query operator list was compiled by Stephan Spencer. This article was written before the demise of the + operator.
Google SERP URL Parameters
Google SERP (search engine results page) URL Parameters are are name/value pairs placed in the query string portion of the Google search URL. The URL parameter most used in our office is the strip parameter in a cache search to eliminate any trace of your pageview in the visited website’s analytics. The SERP URL Parameters article was written before the demise of the + operator.