Why Isn\'t My Regex Negated Set Working in Go?

Regex Expression Negated Set Not Working in Go

A user has encountered an issue where a regular expression with a negated set was not working as expected in Go. The expression, verified on multiple online regex parsers, was intended to match strings that did not end in specific file extensions.

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

However, implementing this expression in Go yielded different results. The user simplified the expression to remove the negated set and obtained desired matches.

Understanding the Issue

Go's regular expression engine, RE2, does not support lookaround operators like the negative lookahead ?! used in the original expression. This absence of lookaround support caused the negated set to be ineffective.

Solution

To ensure that a string ends in a three-character file extension, the user can simplify the expression to:

\.\w{3}$

This expression matches a literal period, followed by three word characters (alphanumeric and underscore), followed by the end of the string. It effectively achieves the same result as the original expression but without relying on lookaround operators.

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