Print the input, the tests are fine. Your code is wrong and it is removing more than a single char from the start and the end sometimes.

Instructions mention nothing about handling numbers differently than letters, but tests seem to expect numbers should be disregarded from trimming. (I was using the c# kata)

Test Failed
  Expected string length 7 but was 6. Strings differ at index 0.
  Expected: "3f39j00", ignoring case
  But was:  "f39j00"

If some tests fail, re-check the worth values whereever you hard-code them into your solution....

I had a typo that drove me nuts and I was convinced the tests were wrong for ages! ;)