Why does a RegExp with global flag give wrong results?

Why does a RegExp with global flag give wrong results?

What is the problem with this regular expression when I use the global flag and the case insensitive flag? Query is a user generated input. The result should be [true, true].
var query = ‘Foo B’;
var re = new RegExp(query, ‘gi’);
var result = [];
result.push(re.test(‘Foo Bar’));
result.push(re.test(‘Foo Bar’));
// result will be [true, false]

var reg = /^a$/g;
for(i = 0; i++ < 10;) console.log(reg.test("a"));


Solution 1:

The RegExp object keeps track of the lastIndex where a match occurred, so on subsequent matches it will start from the last used index, instead of 0. Take a look:

var query = 'Foo B';
var re = new RegExp(query, 'gi');
var result = [];
result.push(re.test('Foo Bar'));


result.push(re.test('Foo Bar'));

If you don’t want to manually reset lastIndex to 0 after every test, just remove the g flag.

Here’s the algorithm that the specs dictate (section


a regular expression match of string
against the regular expression and
returns an Array object containing the
results of the match, or null if the
string did not match The string
ToString(string) is searched for an
occurrence of the regular expression
pattern as follows:

  1. Let S be the value of ToString(string).
  2. Let length be the length of S.
  3. Let lastIndex be the value of the lastIndex property.
  4. Let i be the value of ToInteger(lastIndex).
  5. If the global property is false, let i = 0.
  6. If I < 0 or I > length then set lastIndex to 0 and return null.
  7. Call [[Match]], giving it the arguments S and i. If [[Match]]
    returned failure, go to step 8;
    otherwise let r be its State result
    and go to step 10.
  8. Let i = i+1.
  9. Go to step 6.
  10. Let e be r’s endIndex value.
  11. If the global property is true, set lastIndex to e.
  12. Let n be the length of r’s captures array. (This is the same
    value as’s
  13. Return a new array with the following properties:
    • The index
      property is set to the position of the
      matched substring within the complete
      string S.
    • The input property is set
      to S.
    • The length property is set to
      n + 1.
    • The 0 property is set to the
      matched substring (i.e. the portion of
      S between offset i inclusive and
      offset e exclusive).
    • For each
      integer i such that I > 0 and I ≤ n,
      set the property named ToString(i) to
      the ith element of r’s captures array.

Solution 2:

You are using a single RegExp object and executing it multiple times. On each successive execution it continues on from the last match index.

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You need to “reset” the regex to start from the beginning before each execution:

result.push(re.test('Foo Bar'));
re.lastIndex = 0;
result.push(re.test('Foo Bar'));
// result is now [true, true]

Having said that it may be more readable to create a new RegExp object each time (overhead is minimal as the RegExp is cached anyway):

result.push((/Foo B/gi).test(stringA));
result.push((/Foo B/gi).test(stringB));

Solution 3:

RegExp.prototype.test updates the regular expressions’ lastIndex property so that each test will start where the last one stopped. I’d suggest using String.prototype.match since it doesn’t update the lastIndex property:

!!'Foo Bar'.match(re); // -> true
!!'Foo Bar'.match(re); // -> true

Note: !! converts it to a boolean and then inverts the boolean so it reflects the result.

Alternatively, you could just reset the lastIndex property:

result.push(re.test('Foo Bar'));
re.lastIndex = 0;
result.push(re.test('Foo Bar'));

Solution 4:

Removing global g flag will fix your problem.

var re = new RegExp(query, 'gi');

Should be

var re = new RegExp(query, 'i');

Solution 5:

Using the /g flag tells it to continue searching after a hit.

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If the match succeeds, the exec() method returns an array and updates properties of the regular expression object.

Before your first search:

//is 0

After the first search

//is 8

Remove the g and it exits the search after each call to exec().

Solution 6:

I had the function:

function parseDevName(name) {
  var re = /^([^-]+)-([^-]+)-([^-]+)$/g;
  var match = re.exec(name);
  return match.slice(1,4);

var rv = parseDevName("BR-H-01");
rv = parseDevName("BR-H-01");

The first call works.
The second call doesn’t. The slice operation complains about a null value. I assume this is because of the re.lastIndex. This is strange because I would expect a new RegExp to be allocated each time the function is called and not shared across multiple invocations of my function.

When I changed it to:

var re = new RegExp('^([^-]+)-([^-]+)-([^-]+)$', 'g');

Then I don’t get the lastIndex holdover effect. It works as I would expect it to.