Using positive-lookahead (?=regex) with re2

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.

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.

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

In cases where you want to match a broad pattern, but exclude specific substrings purely in Regex you can use a technique called "Stepwise Exclusion"

This technique involves iteratively refining the regex to exclude specific sequences character by character.

Let's consider an example. Suppose you want to match all email addresses ending with "@google.com", but exclude the specific address "noreply@google.com". Here's how you would construct such a regex using the stepwise exclusion technique:

^(?i)([\w]{1,6}|[a-mo-z0-9_][\w]*|n[a-np-z0-9_][\w]*|no[a-qs-z0-9_][\w]*|nor[a-df-z0-9_][\w]*|nore[a-oq-z0-9_][\w]*|norep[a-km-z0-9_][\w]*|norepl[a-xz0-9_][\w]*)@google\.com

Breakdown of the Pattern

1. (?i): This flag makes the regex case insensitive.
2. [\w]{1,6}: This part matches any email address containing shorter but not complete parts of noreply such as no@google.com
3. [a-mo-z0-9_][\w]*: This part matches any email that starts with any alphanumeric character or underscore (except for n) and ends with @google.com.
4. Each subsequent part of the pattern (e.g., n[a-np-z0-9_][\w]*, no[a-qs-z0-9_][\w]*, etc.) is designed to progressively exclude the characters in "noreply" when they appear in the same sequence.
5. The last part, noreply[\w]*, matches addresses that start with 'noreply' and have additional characters before @google.com.