why wouldn't you just keep the original, like regexp.Regex ?

cc @rsc

How are you printing it? The existing code does exactly what you want: https://go.dev/play/p/R5sPE02FYKJ

package main

import (
	"fmt"
	"regexp"
)

func main() {
	fmt.Println(regexp.MustCompile(`(?i:ab*c|d?e)`))
}

We generate a single regular expression out of multiple smaller expressions. Here's an example:

\s+__\$
command1
command2

Each line is treated as an alternation. The result would be \s+__\$|command1|command2. We do more complicated things too, such as substitutions and concatenation. In the end, we always end up with a list of alternations as above. We then feed these to a library (https://github.com/itchyny/rassemble-go) for final assembly and optimisation.

This process is also recursive in some cases, so the output from rassemble-go may become one of the alternations.

rassemble-go uses regex/syntax to parse and process the expression:

import "regexp/syntax"

r, _ := syntax.Parse("(?i:ab*c|d?e)", syntax.ClassNL|syntax.PerlX)
println(r.String())

The result is (?i:A)(?i:B)*(?i:C)|(?i:D)?(?i:E).

To finalize:

1. the "original" expression may be embedded in another expression and I don't know the position in the string where I would need to insert the flags
2. rassemble-go uses regexp/syntax to parse and manipulate the regular expression, hence the output differs from the one produced by regexp.Regexp

Change https://go.dev/cl/507015 mentions this issue: regexp/syntax: use more compact Regexp.String output

I think this is a simple enough change that it doesn't need a proposal. I sent https://go-review.googlesource.com/c/go/+/507015. It will probably be in Go 1.22 (too late for the upcoming Go 1.21). You can copy the code if you want to use it ahead of then.

Wow, that's amazing @rsc! I wouldn't call that a "small change" but I'm not complaining 😄.