How Can I Achieve Literal Pattern Matching in PostgreSQL LIKE Expressions with Unvalidated User Input?

Literal Pattern Matching in PostgreSQL LIKE Expressions for Unvalidated User Input

When matching user input against a LIKE pattern in PostgreSQL, it's essential to escape special pattern characters like "_" and "%" to ensure literal matching. PostgreSQL requires these characters to be quoted using the backslash () or a custom escape character defined with the ESCAPE clause.

For example, to match "rob" literally, you would need to escape any user input containing underscores or percent signs. However, there's a potential pitfall here: if your user input also contains backslashes, they would also need to be escaped. This can become complex and lead to bugs.

Server-Side Solution

To handle this elegantly, you can leverage PostgreSQL's replace() function to replace special characters with their escaped versions. This approach has several advantages:

• It's performed on the server, eliminating the need for client-side handling.
• It escapes all necessary characters, ensuring consistency.
• It works seamlessly even when the user input contains special characters in addition to underscores and percent signs.

For instance, to search for "rob" literally, you could use the following query:

SELECT * FROM users WHERE name LIKE replace(replace(replace(,'^','^^'),'%','^%'),'_','^_') ||'%' ESCAPE '^'

In this query:

• $1 is a placeholder for the user input.
• replace($1,'^','^^') escapes any caret characters (^) in the input string.
• replace(replace($1,'^','^^'),'%','^%') escapes any percent signs (%) in the input string.
• replace(replace(replace($1,'^','^^'),'%','^%'),'_','^_') escapes any underscores (_) in the input string.
• ||'%' appends a trailing percent sign to the search string for prefix matching.
• ESCAPE '^' specifies "^" as the escape character, allowing you to escape any of the three special characters.

Conclusion

By using server-side replacement and a custom escape character, you can ensure literal pattern matching in PostgreSQL LIKE expressions for unvalidated user input. This approach is robust, avoids injection vulnerabilities, and simplifies the code.

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