URL Encoding Best Practices for Web Developers
Proper URL encoding prevents broken links and security vulnerabilities. Learn which characters must be encoded and how to handle international URLs.
Key Takeaways
- URLs can only contain ASCII characters.
- Spaces become `%20` (or `+` in query strings).
- Different URL parts have different encoding rules:
- Domain names with non-ASCII characters use Punycode encoding.
- A common bug is encoding already-encoded values, turning `%20` into `%2520`.
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Why URL Encoding Matters
URLs can only contain ASCII characters. Special characters, spaces, and non-ASCII characters must be percent-encoded (%XX format) to be safely transmitted. Incorrect encoding causes broken links, XSS vulnerabilities, and data loss.
Characters That Must Be Encoded
Spaces become %20 (or + in query strings). Forward slashes in values must be encoded as %2F. Ampersands in values need %26 to avoid being interpreted as parameter separators.
Component-Specific Encoding
Different URL parts have different encoding rules:
- Path: Encode spaces and special chars, preserve
/. - Query string: Encode spaces as
+, preserve&and=as separators. - Fragment: Encode most special characters.
International Domain Names (IDN)
Domain names with non-ASCII characters use Punycode encoding. For example, mรผnchen.de becomes xn--mnchen-3ya.de. Modern browsers display the Unicode version while transmitting the Punycode version.
Double Encoding
A common bug is encoding already-encoded values, turning %20 into %2520. This happens when a URL passes through multiple encoding layers. Always check if data is already encoded before encoding it again.
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