Regex Tester & Validator

This regex uses a positive lookbehind to capture the first line of text immediately following an "X-Filename" header and a subsequent blank line. It skips the header content itself, which can contain alphanumeric characters and basic symbols, and then extracts everything up to the next line break. It is commonly used in parsing raw HTTP requests, email headers, or custom log formats where the payload or filename is located just after a double newline.