Replace with regex in Golang

You can use capturing groups with alternations matching either string boundaries or a character not _ (still using a word boundary):

var re = regexp.MustCompile(`(^|[^_])\bproducts\b([^_]|$)`)
s := re.ReplaceAllString(sample, `$1.$2`)

Here is the Go demo and a regex demo.

Notes on the pattern:

• (^|[^_]) – match string start (^) or a character other than _
• \bproducts\b – a whole word “products”
• ([^_]|$) – either a non-_ or the end of string.

In the replacement pattern, we use backreferences to restore the characters captured with the parentheses (capturing groups).