How to Work Around Lookarounds in Go Regex?

Error Parsing Regexp: Invalid Perl Syntax (?!

When attempting to use a regular expression in Go, an error is encountered:

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

This error occurs specifically with the following regex:

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

Solution

The issue arises because Go regex doesn't support lookarounds, unlike Perl. Lookarounds are assertions that check the surrounding text without consuming it.

To work around this limitation, use a different approach:

Firstly, compile two separate regular expressions:

first := regexp.MustCompile(`^On\s(.+?)wrote:$`)
second := regexp.MustCompile(`^On.*On\s.+?wrote:`)

Then, perform the following steps:

1. Check if first matches the string.
2. If it does, proceed to step 3. Otherwise, the string matches neither expression, so return true.
3. Check if second matches the string.
4. If it does, the string matches the second expression, so return false. Otherwise, return true.

Alternatively, you can use an optional capturing group to simplify the process:

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

Check for a match and return true if:

• Group 1 ends with On.
• Group 1 doesn't exist or doesn't end with On.

Otherwise, return false.

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