How to Implement Negative Set in Regex for Go: A Guide to Avoiding Unexpected Results

Implementing Negative Set in Regex for Go

The provided regular expression aims to capture file names that do not end in specific file extensions (e.g., htm, html, class, js). However, the implementation in Go using the regexp package yields different results than online regex parsers.

This discrepancy arises because the Go standard library's RE2 regex engine does not support negative lookaheads (i.e., the ?! operator used in the original expression). As a result, it cannot exclude strings ending in the specified file extensions.

To replicate the behavior of the online regex parser, you can consider the following approaches:

• Use a Positive Lookahead (?!.(htm|html|class|js)$): This asserts that the input string does not end with one of the specified file extensions.

<code class="go">re := regexp.MustCompile(`^(.*\.(?!(htm|html|class|js)$))([^.]*)$`)</code>

• Use a Negative Lookbehind (? This asserts that the input string is not preceded by any of the specified file extensions.

<code class="go">re := regexp.MustCompile(`^(.*?)(?<!\.(htm|html|class|js)$)`)</code>

• Remove Negative Set and Use Literal File Extension: If you only need to ensure that the string ends in a three-character file extension, you can simplify the expression to:

<code class="go">re := regexp.MustCompile(`.\w{3}$`)</code>

Note that these expressions may yield different results as they implement the logic in different ways. Choosing the approach that best fits your use case is essential.

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