oystercar ☃ ~
10001 ◯ : coffee
coffee> str = "The quick brown fox jumped over the lazy dog."
'The quick brown fox jumped over the lazy dog.'
coffee> str[1..]
'he quick brown fox jumped over the lazy dog.'
coffee> str[5..]
'uick brown fox jumped over the lazy dog.'
coffee> str[20..]
'jumped over the lazy dog.'
coffee> ending = 25
25
coffee> str[ending..]
'd over the lazy dog.'
coffee> ending = 3
3
coffee> str[-ending..]
'og.'

oystercar ☃ ~ 10001 ◯ : coffee
coffee> str = "The quick brown fox jumped over the lazy dog."
'The quick brown fox jumped over the lazy dog.'
coffee> str[1..]
'he quick brown fox jumped over the lazy dog.'
coffee> str[5..]
'uick brown fox jumped over the lazy dog.'
coffee> str[20..]
'jumped over the lazy dog.'
coffee> ending = 25
25
coffee> str[ending..]
'd over the lazy dog.'
coffee> ending = 3
3
coffee> str[-ending..]
'og.'

Hope that clears it up! :)

the str.length here is superfluous.
str[-ending.length..] is ending would work the same.