Split a string only the at the first n occurrences of a delimiter

Question

I\'d like to split a string only the at the first n occurrences of a delimiter. I know, I could add them together using a loop, but isn\'t there a more straight forward appr

Answer 1

My version, universal, supports RegExp and non-RegExp delimiters. Highly optimized. Tests provided. Why: since other RegExp versions are full of bugs and this is not a trivial function.

Usage:

"a b  c   d".split_with_tail(/ +/,3) = ['a','b','c   d']
"a b  c   d".split_with_tail(' ',3) = ['a','b',' c   d']

Code

String.prototype.split_with_tail = function(delimiter,limit)
{
    if( typeof(limit) !== 'number' || limit < 1 ) return this.split(delimiter,limit);

    var parts = this.split(delimiter,limit+1);
    if( parts.length <= limit ) return parts;
    parts.splice(-2,2);

    limit = Math.floor(limit) - 1; // used later as index, speed optimization; limit can be float ..
    if( delimiter instanceof RegExp ) {
        // adds 'g' flag to any regexp:
        delimiter += '';
        var len = delimiter.lastIndexOf('/');
        delimiter = new RegExp(delimiter.slice(1, len), delimiter.slice(len + 1)+'g');

        len = 0;
        while(limit--) len += parts[limit].length + (delimiter.exec(this))[0].length;
    }
    else {
        var len = limit * (''+delimiter).length;
        while(limit--) len += parts[limit].length;
    }

    parts.push(this.substring(len)); // adds tail, finally
    return parts;
}

Tests

function test(str,delimiter,limit,result) {
    if( JSON.stringify(result) !== JSON.stringify(str.split_with_tail(delimiter,limit)) ) {
        console.log(arguments);
        console.log(str.split_with_tail(delimiter,limit));
        throw "lol";
    }
}
test('',/ +/,undefined,['']);
test('',/ +/,3,['']);
test('a',/ +/,0.1,[]);
test('a',/ +/,1,['a']);
test('a a',/ +/,1,['a a']);
test('a a',/ +/,2.1,['a','a']);
test('a a a',/ +/,2.9,['a','a a']);
test('aaaaa aa a',/ +/,1,['aaaaa aa a']);
test('aaaaa aa a',/ +/,2,['aaaaa', 'aa a']);
test('a a',/ +/,2,['a','a']);
test('a',/ +/,3,['a']);
test('a a',/ +/,3,['a','a']);
test('a a  a',/ +/,3,['a','a','a']);
test('a a  a  a',/ +/,3,['a','a','a  a']);
test('a a  a  a',/ +/,4,['a','a','a','a']);
test('a aa  aaa  ',/ +/,4,['a','aa','aaa','']);
test('a a  a  a',/ +/,2,['a','a  a  a']);
test('a a  a  a',/ +/,1,['a a  a  a']);
test('a a  a  a',/ +/,0,[]);
test('a a  a  a',/ +/,undefined,['a','a','a','a']);
test('a a  a  a',/ +/,-1,['a','a','a','a']);

test('a',' ',3,['a']);
test('aaaaa aa a',' ',2,['aaaaa', 'aa a']);
test('aaaaa  aa  a','  ',2,['aaaaa','aa  a']);
test('a a a',' ',3,['a','a','a']);
test('a a a a',' ',3,['a','a','a a']);
test('a a  a a',' ',3,['a','a',' a a']);
test('a a  a a',' ',2,['a','a  a a']);
test('a a  a a',' ',1,['a a  a a']);
test('a a  a a',' ',0,[]);
test('a a  a a',' ',undefined,['a','a','','a','a']);
test('a a  a a',' ',-1,['a','a','','a','a']);
test('1232425',2,3,['1','3','425']);
console.log("good!");