Matching string that isn't wrapped with certain characters

I am trying to make automatic tag detection and convertion into hyperlinks. Problem is, that it has to be done after the string is run through the following:

htmlspecialchars($string, ENT_QUOTES, "UTF-8");

Now, i.e., the ' symbol is turned into '. The tags are in the form of #[a-Z0-9\-\_]

So, the script considers the encoded special characters as tags because of the #39 part.

How do I match with preg_match so, that it would not consider开发者_如何学C # marks preceded with & mark as tags?

Thank you!

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You have to use a lookbehind assertion to check that the string is not preceded by a &:

Try with this:

"/(?<!&)#[\w-]+/"

The (?<!&) cause the # to match only if it is not preceded by &.

The \w part matches [a-zA-Z0-9_]

You may also want to check if the tag is preceded by a whitespace or is a the start of the string:

"/(:?^|\s)#[\w-]+/"

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Use a Look Behind assertion

(?<!a)b matches a "b" that is not preceded by an "a"

In your case, that would be

(?<!&)#[a-Z0-9\-\_]

Will not match # preceded by &