Proxyman.io - Proxyman is a high-performance macOS app, which enables developers to view HTTP/HTTPS requests from apps and domains.
Endpoints - View and respond to requests on an HTTP endpoint
RegExr - RegExr.com is an online tool to learn, build, and test Regular Expressions.
httpbin(1) - HTTP request and response service
Regulex - Open-source, regular expression parser and visualizer.
HTTP Toolkit - Beautiful, cross-platform & open-source tools to debug, test & build with HTTP(S). One-click setup for browsers, servers, Android, CLI tools, scripts and more.