Different regex behaviour in web page and Chrome extension

Question

I am trying out this regex on this page: http://www.regular-expressions.info/javascriptexample.html

where Regex=(?:http:\/\/)?(?:www.)?facebook\.com\/([\w\-\.]*)?

and Subject String = http://www.facebook.com/xxxxxx

This returns me two matches: http://www.facebook.com/xxxxxx and xxxxxx

The same javascript I have embedded in my chrome extension, however there it shows me only one match: 'http://www.facebook.com/' . Any ideas ? Following is the code:

var re = new RegExp("(?:http:\/\/)?(?:www.)?facebook\.com\/(?:profile\.php\?id=(?=\d.*))?([\w\-]*)?"); var m = re.exec("http://www.facebook.com/xxxxx"); if (m == null) { alert("No match"); } else { var s = "Match at position " + m.index + ":\n"; for (i = 0; i < m.length; i++) { s = s + m[i] + "\n"; } alert(s); }

In your first example, there aren't really two matches - the first entry is the part of the string that matches the pattern. The second entry is the part within the capturing group. — Nathan Osman
– Nathan Osman, Commented Jun 15, 2011 at 23:07
@George : Sorry, but i do not understand. The same is running on that web page and chrome extension. Still the output is different. — Madhur Ahuja
– Madhur Ahuja, Commented Jun 15, 2011 at 23:17

serg · Accepted Answer · 2011-06-16 00:18:23Z

When regexp comes from a string, each backslash should be masked:

var re = new RegExp("(?:http:\\/\\/)?(?:www.)?facebook\\.com\\/(?:profile\\.php\\?id=(?=\\d.*))?([\\w\\-]*)?");

In javascript you can also create regexp patterns without strings:

var re = /(?:http:\/\/)?(?:www.)?facebook\.com\/(?:profile\.php\?id=(?=\d.*))?([\w\-]*)?/;

+1. I was surprised it didn't throw an exception, but the only thing it seems to have a problem with is the \w in [\w\-]. Since that part of the regex is optional anyway, the match succeeds without consuming the xxxxx part.

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