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%20 represents a space octet in percent-encoding. A plus sign represents a space only under the application/x-www-form-urlencoded convention; elsewhere, it may be a literal plus. If you are fixing a query-string bug, the key is to encode the value for the format your receiving API expects, then parse the URL before decoding its data.
What percent-encoding represents
Percent-encoding represents an octet as a three-character sequence: a percent sign followed by two hexadecimal digits. For example, %20 represents octet 0x20, the US-ASCII space. The sequence represents bytes, not an abstract character by itself. For text outside ASCII, the relevant format or scheme determines the character encoding used to produce those bytes.
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RFC 3986 recommends UTF-8 for new URI schemes that carry non-ASCII text, followed by percent-encoding octets that are not in the unreserved set. Hexadecimal letters in percent triplets are case-insensitive, though the RFC recommends uppercase for consistency. RFC 3986, §§ 2.1 and 2.5
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| Representation | Meaning and scope | What a parser may do |
|---|---|---|
%20 |
Percent-encoding for the space octet. | A percent-decoder converts the triplet to the represented octet; interpreting that octet as text depends on the applicable character encoding. |
+ |
Represents a space in application/x-www-form-urlencoded data. In generic URI syntax, plus is not a universal space marker. |
A form-style parser maps plus to space. A parser for another URI component need not. |
%2B |
Percent-encoding for a literal plus sign when plus would otherwise be treated as the form-style space convention. | A matching percent-decoder restores the plus as data. |
For example, a form-style serialization of the value C++ guide could be C%2B%2B+guide: the two literal plus signs are escaped, while the space uses plus. This illustrates the form convention; not every language or library emits identical output in every context. The WHATWG URL Standard defines browser-platform form serialization, and MDN explains the distinction between percent-encoding and form encoding. WHATWG URL Standard; MDN: Percent-encoding
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Does plus mean space in a URL?
No—not in every URL. The plus-for-space behavior belongs to the form-style application/x-www-form-urlencoded convention. A plus in another URI component can remain a plus. This is why the same visible query text can produce different results when one system treats it as form-encoded name/value data and another applies only generic URI parsing. Follow the receiving endpoint’s contract and pair its expected parser with a matching serializer.
Encode the component or value, not the whole URL
A URL contains structural characters as well as data. The question mark introduces the query component, and the hash introduces a fragment. In common key/value query formats, ampersand separates pairs and equals sign separates a key from its value. If one of those characters is part of a value, it must be represented as data according to the chosen convention so it is not mistaken for a delimiter.
There is no single encoding rule for every place in a URL. A slash in a path may separate segments; in a query, RFC 3986 permits slash and question mark as query data. Encoding every reserved character everywhere can therefore alter meaning just as leaving data unescaped can. RFC 3986, §§ 2.2 and 3
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- Whole URL: give a complete URL to a URL parser or builder, not to a function intended to encode one value.
- Path data: encode data within the relevant path segment; preserve separators that are intended to divide segments.
- Query parameter: pass raw key/value data to a query or form serializer instead of concatenating unescaped strings.
In browser JavaScript, the URL API parses URL structure and URLSearchParams works with query parameter pairs using browser URL/form rules. Other languages and frameworks may differ, so verify the API’s behavior and the server’s expected format. MDN: URL API; WHATWG URL Standard
Why API query strings turn plus signs into spaces
A common cause is a mismatch between the sender and receiver: the sender means a literal plus, but the receiver parses the value as form-encoded data and converts plus to space. If the receiver expects form-style parameters, serialize a literal plus as %2B. If it expects a different query format, use that format’s rules instead; plus is not intrinsically a space in generic URI syntax.
Another cause is manually joining strings. For example, inserting raw ampersands or equals signs into a value can make a parser treat them as pair separators or key/value boundaries. Use a serializer for the specific parameter format, and confirm that the endpoint expects key/value pairs at all: RFC 3986 defines generic query syntax but does not require every application to interpret a query as key=value&....
Parse first, then decode
Split a URI into its components and relevant subcomponents before decoding percent-encoded octets. If a program decodes first, %26 becomes & and %3D becomes =; data can then be mistaken for query syntax. RFC 3986 explicitly warns that decoding before parsing can cause decoded octets to be mistaken for delimiters. RFC 3986, § 2.4
Apply encoding once when data crosses into the URL representation, and apply the matching decode after parsing the structure. Encoding already encoded text again can turn percent signs into %25; repeated decoding can make data acquire structural meaning. The exact failure depends on the API stack, so avoid layering unrelated encoders and decoders.
Quick Recap
A practical checklist for fixing a URL-encoding bug
- Identify the input: determine whether you have a complete URL, a path segment, a generic query component, or a form-style parameter value.
- Confirm the receiver’s contract: establish whether the endpoint parses
application/x-www-form-urlencodeddata or uses another query format. - Use a matching serializer: supply raw values to a URL or parameter API rather than assembling query text by hand.
- Check reserved characters: ensure data such as
&,=,?, or#cannot be interpreted as structure in that context. - Inspect the wire value: check whether a space is represented as
%20or form-style+, and whether a literal plus is%2B. - Trace parsing order: verify that the receiver separates URL components and parameter pairs before decoding their values.
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