Using positive-lookahead (?=regex) with re2

Question

Since I'm a bit new with re2, I'm trying to figure out how to use positive-lookahead (?=regex) like JS, C++ or any PCRE style in Go.

Here's some examples of what I'm looking for.

JS:

'foo bar baz'.match(/^[\s\S]+?(?=baz|$)/);

Python:

re.match('^[\s\S]+?(?=baz|$)', 'foo bar baz')

• Note: both examples match 'foo bar '

Thanks a lot.

Answer 1

According to the Syntax Documentation, this feature isn't supported:

(?=re) before text matching re (NOT SUPPORTED)

Also, from WhyRE2:

As a matter of principle, RE2 does not support constructs for which only backtracking solutions are known to exist. Thus, backreferences and look-around assertions are not supported.

Answer 2

You can achieve this with a simpler regexp:

re := regexp.MustCompile(`^(.+?)(?:baz)?$`)
sm := re.FindStringSubmatch("foo bar baz")
fmt.Printf("%q\n", sm)

sm[1] will be your match. Playground: http://play.golang.org/p/Vyah7cfBlH