Inverse Symbolic Calculator (ISC for short) is down indefinitely (and has been down for many years). Wolfram alpha can give answers in simple cases, but from my experience ISC was much more useful. What should I do now?
What would you use instead of this resource now?
On
This is mentioned by Gottfried Helms in a comment, but it really deserves to be a full answer: https://mrob.com/pub/ries/
You'll have to compile and run it yourself (I could swear there used to be a web UI years ago), but it's exactly what you're looking for. The results are also better (i.e. simpler and more plausible) than those provided by ISC in my experience. They're ranked by "complexity", which makes finding the symbolic expression you're looking for a lot simpler.
The site is down indefinitely, however, it is still accessible at the original site (via the Wayback Machine).