Why Does Escaping MySQL Wildcards Require Double Backslashes?

Escaping MySQL Wild Cards: Understanding the Double Escaping Conundrum

When working with user input in PHP and connecting to a MySQL database, escaping special characters is crucial to prevent malicious input. Traditionally, PHP's mysql_real_escape_string() function has been used for this purpose, but it misses characters like % and _, which are wildcards in MySQL. As a result, many developers resort to using addcslashes to escape these characters as well.

However, the unexpected behavior of addcslashes raises questions. Why does it escape _ with a backslash but not " or '? In this article, we'll delve into the nuances of MySQL wild card escaping and explore a solution to correctly handle them.

Understanding LIKE Escaping

Unlike other wildcard characters, % and _ are not wildcards in standard MySQL string literals. They become special characters only in the context of LIKE matching, where they indicate wildcard matching. To prepare strings for LIKE use, an additional layer of escaping, known as LIKE escaping, is required, even with parameterized queries.

In LIKE escaping, _ and % must be escaped, along with the escape character itself (typically the backslash). However, contrary to standard SQL, MySQL uses a backslash for both literal backslash escapes (in string literals) and LIKE escaping.

Double Escaping and the Problem with Backslashes

This dual use of backslashes creates a confusing double escaping scenario. To match a literal percent sign (%), for example, you would need to double-backslash-escape it using LIKE 'something\%'. However, this is incorrect according to ANSI SQL, and depending on the database, you may encounter problems.

A Portable Solution

To resolve this issue and ensure portability, you should override the default escape character in LIKE expressions using the LIKE ... ESCAPE ... construct. Here's a function showcasing this approach:

function like($s, $e) {
    return str_replace(array($e, '_', '%'), array($e.$e, $e.'_', $e.'%'), $s);
}

With this function, you can escape strings for LIKE use while ensuring ANSI SQL compatibility.

Conclusion

While escaping wild cards in MySQL may seem straightforward, the double escaping conundrum can cause confusion. By understanding the intricacies of LIKE escaping and employing a portable solution, you can effectively prevent malicious input and ensure the integrity of your database.

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