@ (at) symbols preg_replace function

Is the "@" symbol sometimes used to surround a PHP regular expression? I'm working with a code base and found this function call:

$content = preg_replace("@(</?[^>]*>)+@", "", $con开发者_运维技巧tent);

I believe it's removing all XML tags from the string but I'm not sure what the "@" symbol in there means.

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Yes, it can be used to wrap the expression. The original author most likely does this because some (or several) expressions contain the "more traditional" / delimiter. By using @, you can now use / without the need to escape it.

You could use:

• /pattern/flags (traditional)
• @pattern@flags
• $pattern$flags
• etc.

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The manual calls them the PCRE "delimiters". Any ASCII symbol (non-alphanumeric) can be used (except the null byte).

Common alternatives to / are ~ and #. But @ is just as valid.

PCRE also allows matching braces like (...) or <...> for the regular expression.

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You can use nearly any punctuation character as a delimiter in PHP regular expressions. See the docs here. Usually, the / is the first choice (and I would have suggested its use here), but if your regex contains slashes, a different character can be useful.

Mostly, I've seen %...%, ~...~ and #...#, but @...@ is OK, too.

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Short answer is yes. The specific symbol used isn't important; it should generally be something that doesn't appear within the pattern. For example, these are all equivalent:

<?php
    preg_replace("@(<\?[^>]*>)+@", "", $content);
    preg_replace("/(<\?[^>]*>)+/", "", $content);
    preg_replace("!(<\?[^>]*>)+!", "", $content);  
?>

The reason the symbol is necessary is because modifiers may be added after the expression. For example, to search case insensitive, you could use:

<?php
   preg_replace("@(<\?[^>]*>)+@i", "", $content);
?>