Question

I am trying to match . \ or / using preg_match in PHP.

I thought this would do it but it’s matching all strings.

$string = '';
$chars = '/(\.|\\|\/)/';
if (preg_match($chars, $string) != 0) {
    echo 'Chars found.';
}

Answer 1

Argument given to preg_match() is string. Strings are automatically escaped by PHP. For example, if you have {\\\\} (backslash) given to the regexp engine, PHP will first parse it creating {\\} (\\ is replaced by \).

Next, regexp engine parses the regexp. It sees {\\} which PHP gave to regexp engine. It sees \ as escape character, so it actually matches \ character which was escaped by \.

In your case, it looks like /(\.|\\|\/)/. PHP gives to regexp engine /(\.|\|\/)/ which is actually either . or |/ (notice that | character was escaped).

Personally, I try to avoid escaping meta-characters, especially with how regexp engine works. I usually use [.] instead, it’s more readable. Your regexp written with this would look like /([.]|\\\\|[/])/.

It’s possible to do few optimizations. While it’s my personal thing, I prefer to use {} as delimiters (yes, you can use pairs of characters). Also, your regexp matches single characters, so you could easily write it as {[.\\\\/]}, which is very readable in my opinion (notice four slashes, it’s needed because both PHP and regexp engine parse backslashes).

Also, preg_match() returns number of matches. It will be always bigger than 0, so you can easily consider it to be boolean and avoid writting == 0. Instead, you can insert ! before string to make it negative. But I think you accidentally reversed condition (it matches if it doesn’t match). Valid code below:

$string = '';
$chars = '{[.\\\\/]}';
if (preg_match($chars, $string)) {
    echo 'Chars found.';
}