2022.06.17
本文按照YAML官方文档1.2.2版本翻译总结而成
The following chapters formally define the syntax of YAML character streams, using parameterized BNF productions. Each BNF production is both named and numbered for easy reference. Whenever possible, basic structures are specified before the more complex structures using them in a “bottom up” fashion.
The productions are accompanied by examples which are presented in a two-pane side-by-side format. The left-hand side is the YAML example and the right-hand side is an alternate YAML view of the example. The right-hand view uses JSON when possible. Otherwise it uses a YAML form that is as close to JSON as possible.
Productions are defined using the syntax production-name ::= term, where a term is either:
An atomic term
A quoted string ("abc"), which matches that concatenation of characters. A single character is usually written with single quotes ('a').A hexadecimal number (x0A), which matches the character at that Unicode code point.A range of hexadecimal numbers ([x20-x7E]), which matches any character whose Unicode code point is within that range.The name of a production (c-printable), which matches that production.
A lookaround
[ lookahead = term ], which matches the empty string if term would match.[ lookahead ≠ term ], which matches the empty string if term would not match.[ lookbehind = term ], which matches the empty string if term would match beginning at any prior point on the line and ending at the current position.
A special production
<start-of-line>, which matches the empty string at the beginning of a line.<end-of-input>, matches the empty string at the end of the input.<empty>, which (always) matches the empty string.
A parenthesized term
Matches its contents.
A concatenation
Is term-one term-two, which matches term-one followed by term-two.
A alternation
Is term-one | term-two, which matches the term-one if possible, or term-two otherwise.
A quantified term:
term?, which matches (term | <empty>).term*, which matches (term term* | <empty>).term+, which matches (term term*).
Note: Quantified terms are always greedy.
The order of precedence is parenthesization, then quantification, then concatenation, then alternation.
Some lines in a production definition might have a comment like:
production-a ::=production-b # clarifying comment
These comments are meant to be informative only. For instance a comment that says # not followed by non-ws char just means that you should be aware that actual production rules will behave as described even though it might not be obvious from the content of that particular production alone.
Some productions have parameters in parentheses after the name, such as s-line-prefix(n,c). A parameterized production is shorthand for a (infinite) series of productions, each with a fixed value for each parameter.
For instance, this production:
production-a(n) ::= production-b(n)
Is shorthand for:
production-a(0) ::= production-b(0)production-a(1) ::= production-b(1)…
And this production:
production-a(n) ::=( production-b(n+m) production-c(n+m) )+
Is shorthand for:
production-a(0) ::=( production-b(0) production-c(0) )+| ( production-b(1) production-c(1) )+| …production-a(1) ::=( production-b(1) production-c(1) )+| ( production-b(2) production-c(2) )+| ……
The parameters are as follows:
Indentation: n or m
May be any natural number, including zero. n may also be -1.
Context: c
This parameter allows productions to tweak their behavior according to their surrounding. YAML supports two groups of contexts, distinguishing between block styles and flow styles.
May be any of the following values:
BLOCK-IN – inside block contextBLOCK-OUT – outside block contextBLOCK-KEY – inside block key contextFLOW-IN – inside flow contextFLOW-OUT – outside flow contextFLOW-KEY – inside flow key context
(Block) Chomping: t
The line break chomping behavior for flow scalars. May be any of the following values:
STRIP – remove all trailing newlinesCLIP – remove all trailing newlines except the firstKEEP – retain all trailing newlinesTo make it easier to follow production combinations, production names use a prefix-style naming convention. Each production is given a prefix based on the type of characters it begins and ends with.
e-
A production matching no characters.
c-
A production starting and ending with a special character.
b-
A production matching a single line break.
nb-
A production starting and ending with a non-break character.
s-
A production starting and ending with a white space character.
ns-
A production starting and ending with a non-space character.
l-
A production matching complete line(s).
X-Y-
A production starting with an X- character and ending with a Y- character, where X- and Y- are any of the above prefixes.
X+, X-Y+
A production as above, with the additional property that the matched content indentation level is greater than the specified n parameter.