String.prototype.match if it only checked the regex at the given index instead of searching the entire string.
; // ['world']; // null
'hello world'.slice(i).match(/^world/) except the resulting match object's
.index property corresponds to the original string, and it doesn't actually slice the string. Most engines optimize taking a substring so this probably isn't particularly valuable in practice, but it was an entertaining exercise and could be useful if you reminisce about these semantics.