Splitting with empty space in Ruby [duplicate]

Question

In both Ruby and JavaScript I can write expression " x ".split(/[ ]+/) . In JavaScript I get somehow reasonable result ["", "x", ""], but in Ruby (2.0.0) i get ["", "x"], which is for me quite counterintuitive. I have problems to understand how regular expressions works in Ruby. Why don't I get the same result as in JavaScript or just ["x"]?

Answer 1

From string#split documentation, emphasis my own:

split(pattern=$;, [limit])

If pattern is a String, then its contents are used as the delimiter when splitting str. If pattern is a single space, str is split on whitespace, with leading whitespace and runs of contiguous whitespace characters ignored.

If pattern is a Regexp, str is divided where the pattern matches. Whenever the pattern matches a zero-length string, str is split into individual characters. If pattern contains groups, the respective matches will be returned in the array as well.

If pattern is omitted, the value of $; is used. If $; is nil (which is the default), str is split on whitespace as if ` ' were specified.

If the limit parameter is omitted, trailing null fields are suppressed. If limit is a positive number, at most that number of fields will be returned (if limit is 1, the entire string is returned as the only entry in an array). If negative, there is no limit to the number of fields returned, and trailing null fields are not suppressed.

So if you were to use " x ".split(/[ ]+/, -1) you would get your expected result of ["", "x", ""]

*edited to reflect Wayne's comment

Answer 2

I found this in the C code for String#split, almost right at the end:

if (NIL_P(limit) && lim == 0) {
long len;
while ((len = RARRAY_LEN(result)) > 0 &&
       (tmp = RARRAY_AREF(result, len-1), RSTRING_LEN(tmp) == 0))
    rb_ary_pop(result);
}

So it actually pops empty strings off the end of the result array before returning! It looks like the creators of Ruby didn't want String#split to return a bunch of empty strings.

Notice the check for NIL_P(limit) -- this accords exactly with what the documentation says, as @dax pointed out.