Why Does `addcslashes` Escape Underscores Differently in MySQL?

Escaping MySQL Wild Cards: Resolving the Underscore Mystery

In MySQL, wildcards and special characters like underscores (_) and percent signs (%) can create escape challenges when user input is sent to the database. The PHP function mysql_real_escape_string handles most escaping needs, but it doesn't extend to _ and %. For this reason, some may resort to using addcslashes to further escape these characters.

However, a perplexing issue arises: when user input includes an underscore, running it through addcslashes does not produce an escaped underscore when the input is retrieved from the database. For instance, if "test_test " ' is sent and then retrieved, the result shows "test_test " '" with an escaped underscore. The question is, why is the underscore escaped differently from other characters that are escaped using the same method?

The answer lies in the fact that _ and % are not wildcards in general MySQL usage. They only become special when used in LIKE statements, where they must be escaped to match their literal forms. To escape characters for use in LIKE statements, a nested escaping process must be employed.

First, the string must be LIKE-escaped within the SQL query. In this context, _ and % are special and must be escaped explicitly, along with the escape character itself. This stage should be performed even when using parameterized queries.

Once the LIKE escaping is complete, the string must be escaped again for regular string use outside of SQL. This is where mysql_real_escape_string comes into play.

The confusion arises because MySQL utilizes a backslash () as the escape character for both stages of escaping. To illustrate, to find an exact match for a percent sign in a LIKE statement, the string would need to be double-backslash-escaped: `LIKE 'something%''. However, ANSI SQL dictates that in string literals, backslashes represent literal backslashes and quote (') characters should be escaped instead of backslashes.

To avoid platform-specific issues, it's recommended to override the default escaping behavior in LIKE statements and define a custom escape character using the LIKE ... ESCAPE ... construct.

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