My code is not correct but it did pass all the tests.
The simplest test case to add: 'ab_ba'. The idea is that all carecters that are not used in the palindrome are special characters.

The description of the cata assumes that you read a describtion of the second one.
Can you please add this part here too:

"When transmitting the Morse code, the international standard specifies that:

"Dot" – is 1 time unit long.
"Dash" – is 3 time units long.
Pause between dots and dashes in a character – is 1 time unit long.
Pause between characters inside a word – is 3 time units long.
Pause between words – is 7 time units long."