Javascript regex matching string ending with number inconsistent

Question

I have the following regex patterns that matches all the 'act' that ends with numbers within a list of URLs.

Regex pattern /\/ch(\d+)\/act(\d+)/gi

Javascript code

pageSrc[1] = "../ch01/index.html";
pageSrc[2] = "../ch01/act01/1.html";
pageSrc[3] = "../ch01/act01/2.html";
pageSrc[4] = "../ch01/act02/1.html";
pageSrc[5] = "../ch01/act02/2.html";
pageSrc[6] = "../ch01/actx/1.html";

var pattern = /\/ch(\d+)\/act(\d+)/gi;
for(var i=0; i<pageSrc.length; ++i){
  var hasAct = pattern.test(pageSrc[i]);
  console.log(hasAct);
}

The expected results and actual results

|   String   | Expected Result |   Actual Result  |
| pageSrc[1] |       false     |     false        |
| pageSrc[2] |       true      |     true         |
| pageSrc[3] |       true      |     *FALSE       |
| pageSrc[4] |       true      |     true         |
| pageSrc[5] |       true      |     *FALSE       |
| pageSrc[6] |       false     |     false        |

Am not sure why pageSrc[3] won't return true. I used the regEx tester on gskinner.com and it worked fine, here is the link http://gskinner.com/RegExr/?344ap

Can anyone help me take a look please? thanks in advance!

Matt Ball · Accepted Answer · 2013-03-14 05:02:26Z

3

Remove the g flag. From the RegExp.test documentation:

As with exec (or in combination with it), test called multiple times on the same global regular expression instance will advance past the previous match.

You don't want a global search when reusing a pattern like this.

> var pageSrc = [];
> pageSrc[1] = "../ch01/index.html";
  pageSrc[2] = "../ch01/act01/1.html";
  pageSrc[3] = "../ch01/act01/2.html";
  pageSrc[4] = "../ch01/act02/1.html";
  pageSrc[5] = "../ch01/act02/2.html";
  pageSrc[6] = "../ch01/actx/1.html";

  var pattern = /\/ch(\d+)\/act(\d+)/i;
  for(var i=0; i<pageSrc.length; ++i){
    var hasAct = pattern.test(pageSrc[i]);
    console.log(i, hasAct);
  }
  0 false
  1 false
  2 true
  3 true
  4 true
  5 true
  6 false

edited Mar 14, 2013 at 5:02

answered Mar 14, 2013 at 4:56

Matt Ball

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Qtax · Accepted Answer · 2013-03-14 04:55:38Z

3

You are using /g. Remove that flag to make it work.

The g flag makes the regex start matching from pattern.lastIndex (the index where the previous match ended), until it fails, and then start at 0.

answered Mar 14, 2013 at 4:55

Qtax

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1 Comment

James Wong Over a year ago

Thanks for replying, your solution works just as Matt's. Since i cannot mark 2 answers, i've up voted yours :) thanks again!

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