fix: detect identifiers in comments using regex, not strings.Fields

[p4l1ly]

With substitution "SomeThing=int", the comment
// foo=SomeThing,OtherThing
was being changed to
// foo=Int,OtherThing.
The capital I is a problem. This fix resolves the problem. I've used regex directly instead of go/parser because it would fail inside comments (didn't try).