ABNF Cheatsheet for HTTP Specs

abnf_cheatsheet.md

ABNF/HTTP Syntax Cheatsheet

Basic ABNF Operators

Operator	Meaning
=	Defines a rule. Example: rule-name = value
/	Logical OR (choice). Example: A / B means "A or B".
*	Zero or more occurrences. Example: *DIGIT means "0 or more DIGITs".
1*	One or more occurrences. Example: 1*DIGIT means "1 or more DIGITs".
[ ]	Optional (zero or one occurrence). Example: [DIGIT] means "optional DIGIT".
()	Grouping. Example: (A / B) SP means "A or B followed by a space".
#	Comma-separated list. Example: #DIGIT means "1+ DIGITs, separated by commas".
ALPHA	Any alphabetic character (A-Z, a-z).
DIGIT	Any numeric digit (0-9).
HEXDIG	Any hexadecimal digit (0-9, A-F).

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Common HTTP/ABNF Terms

Term	Definition
SP	Space (U+0020).
HTAB	Horizontal tab (U+0009).
CR	Carriage return (\r, U+000D).
LF	Line feed (\n, U+000A).
CRLF	Carriage return + line feed (\r\n).
OWS	Optional Whitespace (*( SP / HTAB )).
BWS	Bad Whitespace (used for backward compatibility, trimmed during parsing).
token	Any sequence of visible ASCII characters except separators.
quoted-string	A string enclosed in double quotes ("), allowing escapes with \.
field-name	A token identifying a header field. Example: Content-Type.
field-value	The value of a header field, which can include tokens or a quoted string.

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HTTP-Specific Syntax Examples

Construct	ABNF Definition	Meaning
OWS	*( SP / HTAB )	Zero or more spaces or tabs (optional whitespace).
header-field	field-name ":" OWS field-value OWS	Header field definition (name, colon, value).
method	token	HTTP request method (e.g., GET, POST).
request-line	method SP request-target SP HTTP-version CRLF	First line of an HTTP request.
status-line	HTTP-version SP status-code SP reason-phrase CRLF	First line of an HTTP response.
absolute-URI	Defined in RFC 3986.	Full URI (scheme, host, path, etc.).