Javascript - MultiLine RegExp: lastIndex stuck on newlines?

>> rx = /^(.*)$/mg

>> tx = 'foonnbar'

>> rx.exec(tx)
[foo,foo]
>> rx.lastIndex
3
>> rx.exec(tx)
[,]
>> rx.lastIndex
4
>> rx.exec(tx)
[,]
>> rx.lastIndex
4
>> rx.exec(tx)
[,]
>> rx.lastIndex
4

>> rx.lastIndex = 5
5
>> rx.exec(tx)
[bar,bar]
>> rx.lastIndex
8
>> rx.exec(tx)
null

/^(.*)((rn|r|n)|$)/gm

/^(.*)[rn]*/gm

var rx = /^/gm,
    tx = 'AnBnC',
    m;

while(m = rx.exec(tx)){
    if(!m[0].length && rx.lastIndex > m.index){
        --rx.lastIndex;
    }

    foo();

    if(!m[0].length){
        ++rx.lastIndex;
    }
}

^(.*)$

^(.+)$

^(.*)n?     // remove all r characters beforehand

^(.*)(?:rn|nr|n|r)?  // all possible CR/LF combinations, but *once* at most