function parseModel (val) {
  // Fix https://github.com/vuejs/vue/pull/7730
  // allow v-model="obj.val " (trailing whitespace)
  val = val.trim();
  len = val.length;

  if (val.indexOf('[') < 0 || val.lastIndexOf(']') < len - 1) {
    index$1 = val.lastIndexOf('.');
    if (index$1 > -1) {
      return {
        exp: val.slice(0, index$1),
        key: '"' + val.slice(index$1 + 1) + '"'
      }
    } else {
      return {
        exp: val,
        key: null
      }
    }
  }

  str = val;
  index$1 = expressionPos = expressionEndPos = 0;

  while (!eof()) {
    chr = next();
    /* istanbul ignore if */
    if (isStringStart(chr)) {
      parseString(chr);
    } else if (chr === 0x5B) {
      parseBracket(chr);
    }
  }

  return {
    exp: val.slice(0, expressionPos),
    key: val.slice(expressionPos + 1, expressionEndPos)
  }
}
Parse a v-model expression into a base path and a final key segment.
Handles both dot-path and possible square brackets.

Possible cases:

- test
- test[key]
- test[test1[key]]
- test["a"][key]
- xxx.test[a[a].test1[key]]
- test.xxx.a["asa"][test1[key]]