Validating browser upload file name and extension with simple regex

Question

I got the regexp right. Works perfectly for Firefox ONLY. How would i make this cross browser, cross platform manner. Since it is file name and extension validation you are right i am using File Upload control.

^[a-zA-Z0-9_\.]{3,28}(.pdf|.txt|.doc|.docx|.png|.gif|.jpeg|.jpg|.zip|.rar)$

matches File name must not be empty[ 3, 28 characters long].

Extension must be within the group.

When this works superb in forefox i assume because the fileUpload.value = Filename.extension in firefox. It awfully fails in Google chrome and IE. I am using the above with .net Regular Expression validator and ClientScript enabled.

I know how to validate it on server, so please no server side solutions.

note:

Google chrome:

Provides the fileupload control value as c:\fakePath\filename.extension

IE:

Provides the Full path.

please provide some examples of inputs you try to match and where exactly it fails — bw_üezi
– bw_üezi, Commented May 12, 2011 at 6:22

bw_üezi · Accepted Answer · 2011-05-12 06:45:20Z

3

You can't use the ^ to start with if you sometimes have a full path but are only interested in the filename. The dot of the filending should be escaped.

You could try something like this:

[^\\/]{3,}\.(pdf|txt|doc|docx|png|gif|jpeg|jpg|zip|rar)$

As it looks you get only the file with Firefox but the full path with other browsers.
I'd always add a prefix / to your string and than validate the last part after the last fileseprator / or \.

This example uses lookahead to check the fileseparator (or manually added /) before the file and also allows the check of max 28 char for filename. see this online regex tester:

(?<=[\\/])[\w\.]{3,28}\.(?:pdf|txt|doc|docx|png|gif|jpeg|jpg|zip|rar)$

edited May 12, 2011 at 6:45

answered May 12, 2011 at 6:21

bw_üezi

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1 Comment

Deeptechtons Over a year ago

thanks for the valuable explanation. this regex works across browsers now.

Denis de Bernardy · Accepted Answer · 2011-05-12 11:02:29Z

1

As things stand, your regex validates garbage like the following:

....pdf
____pdf

It also rejects perfectly valid files:

i.jpg
my-pic.jpg
pic.JPG

The easiest is to validate things in multiple steps:

Extract the extension:
```
\.[a-zA-Z]{3,4}$
```
Lowercase the extension and validate it against an array of acceptable values.
Optionally validate the file's name (though I'd recommend cleaning it instead):
```
[a-zA-Z0-9_-]+(?:\.[a-zA-Z0-9_-]+)*
```

answered May 12, 2011 at 11:02

Denis de Bernardy

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