Regex for checking presence and absence of keywords in User Agent String

Question

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()); switch(true) { case (is_not_tablet_bool): return true; break; } ``` I'm trying to craft a regex that will do something close to the opposite of the above i.e. - ensure that 'android' keyword is present in string, and at the same time - ensure that 'mobile' keyword is absent from string Thanks for the anticipated assistance.

Accepted Answer

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):

Replace a matching negative string that will fail afterwards:

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

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;
```

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 of `switch(true)` statement and simply replace it by `return is_not_tablet_bool;`.

Regex for checking presence and absence of keywords in User Agent String

Solution

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