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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
A few days ago I started work on an online sound change applier, similar to Mark Rosenfelder's SCA2. The syntax is slightly different, which allows you to write some rules more compactly or clearly, but it is feature-rich enough to be a full replacement for SCA2. Thorough documentation is available by pressing the "Help" button, and all the code is open-source and is available on GitHub under the GNU GPLv3 license. I will add more features as people request them or as I get new ideas.