Short answer

There is no official documentation so you should look at user created documentation.

Extended answer

The Gmail documentation:

• for end-users can be found at http://support.google.com
• for developers can be found at http://developers.google.com/gmail

Both places point readers looking for advanced search operators to https://support.google.com/mail/answer/7190?hl=en

Neither of previous sites states the effects of special characters with other characters but Sensesul made a great blog post about that. See - How Gmail Filter Email-Matching Works dated on Wednesday, July 14, 2010.

According to Aʟ E. on this answer, the Gmail Search official help article used to say that

Gmail doesn't recognize special search characters like square brackets, parentheses, currency symbols, the ampersand, the pound sign, and asterisks

But the above statement was removed from the current version.

String quote enclosed

"" are used to search for an exact word or phrase. The official help article about it doesn't mention that special characters are ignored, but they are.

Regarding the pipe as search operator

The oldest dated post/web page that I found that mention the use of the pipe character | as search operator is Gmail search operators work in filters, too by Gina Trapani dated on 4/10/06 12:30pm, but this mentions the use of double pipe ||

Another old post that mentions the use of single pipe | is Ten Must-Have Gmail Filters Available for Download by Gina Trapani dated on 3/11/09 9:00am.

See also

• Create Gmail filter that contains a special character dated on asked Dec 5 '13 at 18:44