Why Does My Regex Negated Set Fail in Go?

Regex Expression Negated Set Not Working in Go

In this post, we explore a regex expression that fails to match strings in Golang as it does in online regex parsers. The issue stems from the negated set being unsupported in Go's standard library's regex engine, RE2.

The regular expression in question is designed to match strings that:

• End with a file extension (e.g., ".htm", ".html", ".class", ".js") as a negative set (i.e., do not end with these extensions).
• End with any character not mentioned in the negative set and not a period (.)

Implementing this expression in Golang doesn't produce the same matches as the online regex parser demonstrates because RE2, used by Golang's standard library, lacks support for lookaround operators like the negative lookahead operator ("?!").

To fix this issue, we have two options:

• Use a lookaround-supporting library: We can implement the negative lookahead using a library that supports lookaround, such as the "github.com/google/re2/regexp" package.
• Simplify the expression: If all we need is to ensure the string ends with a three-character file extension, we can simplify the expression to ".w{3}$" which matches a literal period followed by three characters at the end of the string.

By using either of these approaches, we can achieve the desired matching behavior in Golang.

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