Regex - check if input still has chances to become matching

Question

We\'ve got such regexp:

var regexp = /^one (two)+ three/;

So only string like \"one two three\" or \"one two three four\

Answer 1

Your original question was simply testing a string's placement within another, specifically, the start. The fastest way to do that would be to use substr on the match string, followed by indexOf. I've updated my original answer to reflect that:

function check(str){
  return 'one two three'.substr(0, str.length) === str;
};
console.log(check('one'));           // true
console.log(check('one two'));       // true
console.log(check('one two three')); // true
console.log(check('one three'));     // false

If you need case-insensitivity, it's still fastest to simply toLowerCase the match & input strings. (If interested, here's a jsperf of substr, indexOf and RegExp testing for the start of a string, case-insensitively: http://jsperf.com/substr-vs-indexof-vs-regex )