Regex Tester & Validator

This regex uses a negative lookahead to assert that the current position is not immediately followed by a character matching the `A-z0-9_` character class. The `A-z` range broadly matches alphanumeric characters, underscores, and several ASCII symbols like `[\]^_`. Because it is a zero-width assertion, it validates the absence of these characters without consuming any text. It is typically used to enforce a custom negative word boundary in text parsing or validation tasks.