Regular Expression Test does not return the same dependent quotation marks

Question

I have a weird case with regex in javascript:

var re = /[^\s]+(?:\s+|$)/g; re.test('foo'); // return true re.test("foo"); // return false 

Is the regular expression type sensitive? My first goal is to extract all the words (separated by one or more spaces) lines.

Thank you for your help.

Julien

Answer 1

When using the g flag in a Javascript regular expression, it will track where the last match was found, and start searching from that index the next time you try to find a match.

Between the two calls to re.test() look at re.lastIndex to find out what I'm talking about.

For instance:

 var re = /[^\s]+(?:\s+|$)/g; re.test('foo'); // return true re.lastIndex; // 3 re.test("foo"); // return false 

You will notice that the type of quotation marks you use does not matter, re.test('foo'); re.test('foo'); re.test('foo'); re.test('foo'); will have the same behavior.

If you want the regular expression to start from scratch, you can either remove the global flag from your regular expression or set re.lastIndex to 0 after each attempt to find a match, for example:

 var re = /[^\s]+(?:\s+|$)/g; re.test('foo'); // return true re.lastIndex = 0; re.test("foo"); // return true 

The alternative comment noted by Blender in the comments can be explained because lastIndex automatically set to 0 when the match fails, so the next attempt after failure will succeed.