PHP regular expression to detect if url is to an image file

Question

I'm very new to regular expressions and need a little help with something complicated.

I have a list of URLs that may as well be in an array that would look like this:

$urls = array(
    "http://example.com/page.php",
    "http://example.com/page.php?key=value",
    "http://example.com/image.jpg",
    "http://example.com/image.jpg?key=value" ...

I want to loop over the array (which is simple enough with foreach) and for each string return true if the URL points to a file that is an image. I have the following regular expression:

"#\.(jpg|jpeg|gif|png)$# i"

... but it seems to only return true is the string ends in one of the given image extensions. I need to compensate for two factors: 1. if the string has a URL query string on the end of it (ie. ?key=value) and whether the extenion (eg. jpg) is actually part of the query string to a non-image file, for example:

http://example.com/page.php?image=file.jpg

Should return false because the URL is pointing to a PHP file, not a jpg

Thank you for any help!

Just an FYI: Unless you actually retrieve it, you cannot know that page.php does not return an image — Sjon
– Sjon, Commented Apr 7, 2016 at 7:52
that's ok, i can check the content-type as a fallback, thanks! — Peter Cullen
– Peter Cullen, Commented Apr 7, 2016 at 7:53
@Sjon: a happy middle ground might be a HEAD request, trusting the web server to assign the correct Content-Type. OP: you won't have a Content-Type unless you make a request, is what he's saying. — Amadan
– Amadan, Commented Apr 7, 2016 at 7:53
Well, I do not like the one-regex solution here: '~^(?!.*\?.*(\.(?:jpg|jpeg|gif|png)\b)).*(?1)(?:$|\?)~' — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Apr 7, 2016 at 7:55
Check tools.ietf.org/html/rfc3986#page-50 on how to properly parse a URL. In your case $6 must end in your declared extensions. — apokryfos
– apokryfos, Commented Apr 7, 2016 at 7:59

Kaspar Lee · Accepted Answer · 2016-04-07 08:42:41Z

8

Complete RegEx Version

Actually, here is a complete RegEx version:

^[^?]*\.(jpg|jpeg|gif|png)

Live Demo on Regex101

How it works:

^[^?]*                  # Removes ?foo=bar&baz=foo
\.(jpg|jpeg|gif|png)    # Image Extension

The first part selects everything up to the ?.... It is the RegEx equivalent to selecting the first item from explode('?', $str). The second part is the same as yours, with the $ removed (since the end of the string may be variables after the ?)

To deal with the following cases of unusual File Extensions like:

test.jpgfoo
test.pngbar
test.jpg.nope
image.jpg-test.php
image.jpg_test.php

Add a Negative Lookahead to the end, (?![\w.\-_]):

^[^?]*\.(jpg|jpeg|gif|png)(?![\w.\-_])

This will make sure there are no letters, another extension, ., - or _ after the accepted file extension. If there are, the RegEx will fail

Live Demo on Regex101

This RegEx will do what you need, if you do as @DevilaN said and explode('?', $str):

\.(jpg|jpeg|gif|png)(\?.*)?$

Live Demo on Regex101

edited Apr 7, 2016 at 8:42

answered Apr 7, 2016 at 7:57

Kaspar Lee

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5 Comments

Kaspar Lee Over a year ago

@DevilaN I added an update for those unusual cases. Just put (?!\w) at the end, to make sure there are no more letters after the accepted file extension

Kaspar Lee Over a year ago

@DevilaN Thanks, fixed that too!

Kaspar Lee Over a year ago

@DevilaN Seriously! ;) I don't even think that is a valid File Extension, but I have fixed it, as well as any with _! Any others?

DevilaN Over a year ago

Yup. example.com/test.png+file.php example.com/test.png~file.php and many many others Generally read section 2: tools.ietf.org/html/rfc3986#section-2 :)

Kaspar Lee Over a year ago

@DevilaN Wow! That'll take ages to include! If the OP wants to do that, he can add then to the Negative Lookahead himself. ;). Wait, I now see how your solution makes more sense!

Kaspar Lee · Accepted Answer · 2020-03-04 22:37:54Z

Your regular expression is ok, but you need to get rid of ?something=something Just explode("?", $string); and use first part which will contain url with filename only. Then proceed with your normal regex.

$urls = array(
    "http://example.com/page.php",
    "http://example.com/page.php?key=value",
    "http://example.com/image.jpg",
    "http://example.com/image.jpg?key=value"
);

function isImage($l) {
    $arr = explode("?", $l);
    return preg_match("#\.(jpg|jpeg|gif|png)$# i", $arr[0]);
}
foreach ($urls as $url) {
    echo $url . ": " .(isImage($url) ? "true" : "false") . "\n";
}

And the result is:

http://example.com/page.php: false
http://example.com/page.php?key=value: false
http://example.com/image.jpg: true
http://example.com/image.jpg?key=value: true

If you want pure regular expression solution then:

function isImage($l) {
    return preg_match("/^[^\?]+\.(jpg|jpeg|gif|png)(?:\?|$)/", $l);
}

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