How Can I Match Strings Without Specific File Extensions in Go\'s Regex Engine?

Understanding Negated Sets in Go's Regex Engine

In Go, the standard library's regular expression engine (RE2) lacks support for lookarounds, including the negative lookahead operator ?! as used in the provided regular expression:

^(.*\.(?!(htm|html|class|js)$))?[^.]

This regex aims to match strings that do not end with specific file extensions. However, in Go, it does not function as expected due to the lack of lookaround support.

Alternative Solutions

Instead of relying on a negated set, there are alternative solutions to handle this scenario in Go:

1. Verify File Extension Directly:
   Replace the negated set with a direct check for the desired file extensions:

   re.MustCompile(`(type1|type2)_(\d+)\.(csv|ini)`)

2. Remove Trailing Period:
   If the goal is to ensure the string ends with a three-character file extension, without any additional characters, a simplified expression can be used:

   re.MustCompile(`\.\w{3}$`)

Matching Behavior

By removing the negated set or using an alternative syntax, Go's regex engine will accurately match the desired strings as expected.

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