/* An FTL file defines a Resource consisting of Entries. */
Resource            ::= (Entry | blank_block | Junk)*

/* Entries are the main building blocks of Fluent. They define translations and
 * contextual and semantic information about the translations. During the AST
 * construction, adjacent comment lines of the same comment type (defined by
 * the number of #) are joined together. Single-# comments directly preceding
 * Messages and Terms are attached to the Message or Term and are not
 * standalone Entries. */
Entry               ::= (Message line_end)
                      | (Term line_end)
                      | CommentLine
Message             ::= Identifier blank_inline? "=" blank_inline? ((Pattern Attribute*) | (Attribute+))
Term                ::= "-" Identifier blank_inline? "=" blank_inline? Pattern Attribute*

/* Adjacent comment lines of the same comment type are joined together during
 * the AST construction. */
CommentLine         ::= ("###" | "##" | "#") ("\u0020" comment_char*)? line_end
comment_char        ::= any_char - line_end

/* Junk represents unparsed content.
 *
 * Junk is parsed line-by-line until a line is found which looks like it might
 * be a beginning of a new message, term, or a comment. Any whitespace
 * following a broken Entry is also considered part of Junk.
 */
Junk                ::= junk_line (junk_line - "#" - "-" - [a-zA-Z])*
junk_line           ::= /[^\n]*/ ("\u000A" | EOF)

/* Attributes of Messages and Terms. */
Attribute           ::= line_end blank? "." Identifier blank_inline? "=" blank_inline? Pattern

/* Patterns are values of Messages, Terms, Attributes and Variants. */
Pattern             ::= PatternElement+

/* TextElement and Placeable can occur inline or as block.
 * Text needs to be indented and start with a non-special character.
 * Placeables can start at the beginning of the line or be indented.
 * Adjacent TextElements are joined in AST creation. */
PatternElement      ::= inline_text
                      | block_text
                      | inline_placeable
                      | block_placeable
inline_text         ::= text_char+
block_text          ::= blank_block blank_inline indented_char inline_text?
inline_placeable    ::= "{" blank? (SelectExpression | InlineExpression) blank? "}"
block_placeable     ::= blank_block blank_inline? inline_placeable

/* Rules for validating expressions in Placeables and as selectors of
 * SelectExpressions are documented in spec/valid.md and enforced in
 * syntax/abstract.js. */
InlineExpression    ::= StringLiteral
                      | NumberLiteral
                      | FunctionReference
                      | MessageReference
                      | TermReference
                      | VariableReference
                      | inline_placeable

/* Literals */
StringLiteral       ::= "\"" quoted_char* "\""
NumberLiteral       ::= "-"? digits ("." digits)?

/* Inline Expressions */
FunctionReference   ::= Identifier CallArguments
MessageReference    ::= Identifier AttributeAccessor?
TermReference       ::= "-" Identifier AttributeAccessor? CallArguments?
VariableReference   ::= "$" Identifier
AttributeAccessor   ::= "." Identifier
CallArguments       ::= blank? "(" blank? argument_list blank? ")"
argument_list       ::= (Argument blank? "," blank?)* Argument?
Argument            ::= NamedArgument
                      | InlineExpression
NamedArgument       ::= Identifier blank? ":" blank? (StringLiteral | NumberLiteral)

/* Block Expressions */
SelectExpression    ::= InlineExpression blank? "->" blank_inline? variant_list
variant_list        ::= Variant* DefaultVariant Variant* line_end
Variant             ::= line_end blank? VariantKey blank_inline? Pattern
DefaultVariant      ::= line_end blank? "*" VariantKey blank_inline? Pattern
VariantKey          ::= "[" blank? (NumberLiteral | Identifier) blank? "]"

/* Identifier */
Identifier          ::= [a-zA-Z] [a-zA-Z0-9_-]*

/* Content Characters
 *
 * Translation content can be written using any Unicode characters. However,
 * some characters are considered special depending on the type of content
 * they're in. See text_char and quoted_char for more information.
 *
 * Some Unicode characters, even if allowed, should be avoided in Fluent
 * resources. See spec/recommendations.md.
 */
any_char            ::= [\\u{0}-\\u{10FFFF}]

/* Text elements
 *
 * The primary storage for content are text elements. Text elements are not
 * delimited with quotes and may span multiple lines as long as all lines are
 * indented. The opening brace ({) marks a start of a placeable in the pattern
 * and may not be used in text elements verbatim. Due to the indentation
 * requirement some text characters may not appear as the first character on a
 * new line.
 */
special_text_char   ::= "{"
                      | "}"
text_char           ::= any_char - special_text_char - line_end
indented_char       ::= text_char - "[" - "*" - "."

/* String literals
 *
 * For special-purpose content, quoted string literals can be used where text
 * elements are not a good fit. String literals are delimited with double
 * quotes and may not contain line breaks. String literals use the backslash
 * (\) as the escape character. The literal double quote can be inserted via
 * the \" escape sequence. The literal backslash can be inserted with \\. The
 * literal opening brace ({) is allowed in string literals because they may not
 * comprise placeables.
 */
special_quoted_char ::= "\""
                      | "\\"
special_escape      ::= "\\" special_quoted_char
unicode_escape      ::= ("\\u" /[0-9a-fA-F]{4}/)
                      | ("\\U" /[0-9a-fA-F]{6}/)
quoted_char         ::= (any_char - special_quoted_char - line_end)
                      | special_escape
                      | unicode_escape

/* Numbers */
digits              ::= [0-9]+

/* Whitespace */
blank_inline        ::= "\u0020"+
line_end            ::= "\u000D\u000A"
                      | "\u000A"
                      | EOF
blank_block         ::= (blank_inline? line_end)+
blank               ::= (blank_inline | line_end)+