Common characters
Why URLs need escaping
A URL may only carry a limited set of ASCII characters. Everything else — spaces, accents, non-Latin scripts, and the reserved characters like ? & = # — has to be written as %XX, one pair per byte of UTF-8.
A space is not always %20
In a path a space is %20. In a query string, form submissions traditionally write it as +. Both decode back to a space, but they are not interchangeable everywhere, so check which part of the URL you are building.
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Frequently asked questions
Q. Why does one character become several %XX pairs?
Because encoding works on bytes, not characters. A character outside ASCII takes two to four bytes in UTF-8, and each byte gets its own pair.
Q. Should I encode the whole URL?
No. Encoding the whole thing would escape the slashes and colon that give the URL its structure. Encode only the individual values you are dropping into it.
Q. Which characters are reserved?
The ones with a job in the syntax — : / ? # [ ] @ ! $ & ' ( ) * + , ; =. Encode them whenever they appear inside a value rather than as separators.