error parsing regexp: invalid or unsupported Perl syntax: `(?!`

Question

When I try this regex in golang I'm getting regex parsing error.

error parsing regexp: invalid or unsupported Perl syntax: (?!

regexp.MustCompile("^(?!On.*On\\s.+?wrote:)(On\\s(.+?)wrote:)$"),

Can someone tell me why its not working and help me to fix this issue?

Thanks

The go regexp package uses RE2 syntax, not PCRE https://github.com/google/re2/wiki/Syntax — JimB, Aug 13 '16 at 14:53
`(?!re) before text not matching re (NOT SUPPORTED)` from the page [here](https://github.com/google/re2/wiki/Syntax) — SomeDude, Aug 13 '16 at 14:57
Use `regexp.MustCompile("^On\\s(.+?)wrote:$")` and `regexp.MustCompile("^On.*On\\s.+?wrote:")` and check if the first one is true and the second is false. — Wiktor Stribiżew, Aug 13 '16 at 16:38
I have this same issue, and am trying to reproduce it. It would help if I knew what this is supposed to match, so that I can apply the solution to my use case. — Mike Warren, Aug 30 '18 at 15:49
@MikeWarren Go regex does not support lookarounds, thus, in each case, workarounds may be unique. — Wiktor Stribiżew, Mar 05 '19 at 00:36

Wiktor Stribiżew · Accepted Answer · 2020-12-09T20:04:44.150

Go regex does not support lookarounds.

As a workaround, you may use

regexp.MustCompile(`^On\s(.+?)wrote:$`)

and

regexp.MustCompile(`^On.*On\s.+?wrote:`)

and check if the first one matches the string and the second does not.

You could also add an optional capturing group (.*On)?

regexp.MustCompile(`^On(.*On)?\s.+?wrote:`)

and check if there is a match and return true if the Group 1 ends with On - if yes, return false, else true.

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