Regex For Checking Presence And Absence Of Keywords In User Agent String

I'm using a Regex in Javascript to sniff the User Agent string. Here is some pseudo code below: is_not_tablet_bool = /android.+mobile/i.test(navigator.userAgent.toLowerCase()); swi

Solution 1:

Negative lookahead

Regular expressions have a construct called negative lookahead which matches a string in the regular expression without capturing the lookahead part:

Your regular expression should be written this way:

/android(?!.*mobile)/i

This will match any string that contains word android that is not followed by word mobile with ignored case. This also means that you can remove the toLowerCase call.

Addition: Negative lookbehind

In case you need only match those strings that have word android in them but lack mobile (either before or after) then a combination of negative lookaheads and lookbehinds will do.

/(?<!mobile.*)android(?!.*mobile)/i

But the problem is that Javascript doesn't support negative lookbehinds. So you have to employ a different trick that will help you determining that situation.

There are several possibilities of which following seem to be most interesting (and last one useful):

1. Replace a matching negative string that will fail afterwards:

   var nav = navigator.userAgent.replace(/mobile.*android/, "fail" );
   return/android(?!.*mobile)/i.test(nav);
   

2. Use two lookaheads. One on normal string and the other on reversed one while also having a reversed regular expression:

   var nav = navigator.userAgent;
   var after = /android(?!.*mobile)/i.test(nav);
   var before = /diordna(?!.*elibom)/i.test(nav.split("").reverse().join(""));
   return before && after;
   

3. Simplicity is key. Two simple regular expressions would do the trick just fine as well:

   var nav = navigator.userAgent;
   return/android/i.test(nav) && !/mobile/i.test(nav);
   

---

Note: I'm not sure whether your code is actual code, because if it is I would strongly recommend you reconsider the use ofswitch(true)statement and simply replace it byreturn is_not_tablet_bool;.

Solution 2:

I don't think using regular expressions makes this task any easier. I would simply check for the inclusion and exclusion of those strings using the String.indexOf(...) method:

functionisNotAndroidMobile(userAgentString) {
  var ua = userAgentString.toLowerCase();
  return (ua.indexOf('android')>=0) && (ua.indexOf('mobile')===-1);
}