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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
I'm not sure what app academy is, but LeetCode will only cover recursion in that list. I think LeetCode has a few regex questions, but not many. This is a good resource I've looked at for learning regex: https://github.com/ziishaned/learn-regex. Remember, if you don't actually practice it, it won't stick.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.