How to Work Around Lookarounds in Go Regular Expressions?

Error Parsing Regex: Invalid Perl Syntax - Understanding the Issue and Finding a Workaround

When attempting to compile the regular expression "^(?!On.*On\s. ?wrote:)(On\s(. ?)wrote:)$" in Go, a common error encountered is:

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

This error stems from the fact that Go does not support lookarounds, a syntax feature available in Perl regular expressions that allows for negative lookaheads like (?!.

Understanding Lookaround Syntax

In regular expressions, a lookaround is a metacharacter that asserts a condition without consuming any characters from the input string. A negative lookahead, denoted by (?!, asserts that the following expression should not match at the current position.

Go Regex Workaround

As Go does not support lookarounds, the above regular expression cannot be used directly. Instead, a workaround using multiple regular expressions and conditional checks is required:

r1 := regexp.MustCompile(`^On\s(.+?)wrote:$`)
r2 := regexp.MustCompile(`^On.*On\s.+?wrote:`)
match1 := r1.MatchString(inputString)
match2 := r2.MatchString(inputString)
if match1 && !match2 {
    // The string does not contain "On ... On" but contains "On ..."
   // Handle match
}

Alternatively, you could use an optional capturing group and check the contents of the group after a successful match:

r := regexp.MustCompile(`^On(.*On)?\s.+?wrote:`)
match := r.FindStringSubmatch(inputString)
if match != nil {
   // Handle match
   if match[1] != "" {
      // Group 1 ends with "On"
   }
}

Additional Notes

• Use the regexp.MustCompile function to compile the regular expression patterns, as it panics on invalid expressions, simplifying error handling.
• The provided workarounds rely on the assumption that the input string follows the specified format. Adjust the regular expressions accordingly if the format varies.

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