Support non-regexp search queries smartly (ie if query isn't a valid regexp)

Created by: sqs

Some users are initially confused or annoyed by Sourcegraph's search syntax, in that it interprets queries as regexps by default. There is an opportunity for us to treat queries as "regexps-if-they-are-valid-regexps-otherwise-do-some-smart-thing".

A couple examples: "as a user, if I ____"

• paste myFuncName( into the query box, I obviously would prefer it to auto-escape the ( -- this is easy (in theory) because as-is it is not a valid regexp, so we can interpret it as a literal string
• type file:*.go, I intend to look for matches in Go files (ie interpret my file: query as a glob and translate it into file:\.go$) -- again, this is easy because *.go is not a valid regexp, so we can munge it

The risk is that it muddies users' understanding of what input is expected. We can mitigate this by telling the users that we auto-corrected their query, as in:

One other possibility (IMO not as good as the one above) is to show instant feedback when there are errors: