String#split strange behavior [duplicate]

Question

I observed a strange behavior of the split method on a String.

"1..2".split('..')      # => ['1', '2']
"1..2".split('..', 2)   # => ['1', '2']

"..2".split('..')       # => ['', '2']
"..2".split('..', 2)    # => ['', '2']

Everything like expected, but now:

"1..".split('..')       # => ['1']
"1..".split('..', 2)    # => ['1', '']

I would expect the first to return the same that the second.

Does anyone have a good explanation, why "1..".split('..') returns an array with just one element? Or is it an inconsistency in Ruby? What do you think about that?

Answer 1

According to the Ruby String documentation for split:

If the limit parameter is omitted, trailing null fields are suppressed.

Regarding the limit parameter, the Ruby documentation isn't totally complete. Here is a little more detail:

• If limit is positive, split returns at most that number of fields. The last element of the returned array is the "rest of the string", or a single null string ("") if there are fewer fields than limit and there's a trailing delimiter in the original string.

Examples:

"2_3_4_".split('_',3)
=> ["2", "3", "4_"]

"2_3_4_".split('_',4)
=> ["2", "3", "4", ""]

• If limit is zero [not mentioned in the documentation], split appears to return all of the parsed fields, and no trailing null string ("") element if there is a trailing delimiter in the original string. I.e., it behaves as if limit were not present. (It may be implemented as a default value.)

Example:

"2_3_4_".split('_',0)
=> ["2", "3", "4"]

• If limit is negative, split returns all of the parsed fields and a trailing null string element if there is a trailing delimiter in the original string.

Example:

"2_3_4".split('_',-2)
=> ["2", "3", "4"]

"2_3_4".split('_',-5)
=> ["2", "3", "4"]

"2_3_4_".split('_',-2)
=> ["2", "3", "4", ""]

"2_3_4_".split('_',-5)
=> ["2", "3", "4", ""]

It would seem that something a little more useful or interesting could have been done with the negative limit.