Word4 is an iOS/Android app built with Ionic Framework (http://ionicframework.com/) that allows you to search for words that match a given pattern.
There is also a website https://word4.binary.ink for the search (how original?!). The website is built as an experiment in learning ReactJS. The backend API is built using Go (golang).
For example searching:
“w_rd” will yield results
Even though this was built using Ionic Framework. I felt more at ease just using the underlying libraries ui-router and AngularJS.