Aether Cheatsheet

Quick syntax lookup. See AETHER.md for full explanations.

File skeleton

@grammar "name"        ; required, must be first
@root rule_name         ; required, must be second
@skip TOKEN             ; optional -- default is @skip SPACE
@noskip                 ; optional -- mutually exclusive with @skip
@case_insensitive       ; optional

; comments run to end of line

TOKEN_NAME := ...        ; ALL_CAPS -- lexer, matched by maximal munch
rule_name  := ...        ; snake-case/kebab-case -- parser, ordered choice

Operators

Syntax	Meaning	Valid in
`a b`	sequence	tokens & rules
<code>a \| b</code>	ordered choice, first match wins	tokens & rules
`a*`	zero or more	tokens & rules
`a+`	one or more	tokens & rules
`a?`	zero or one	tokens & rules
`a{3}`	exactly 3	tokens & rules
`a{3,}`	3 or more	tokens & rules
`a{3,7}`	3 to 7	tokens & rules
`&a`	positive lookahead (consumes nothing)	tokens & rules
`!a`	negative lookahead (consumes nothing)	tokens & rules
`( a )`	grouping	tokens & rules
`name:a`	named capture	rules only
`~a`	suppress `@skip` splicing before this term	rules only
`@indent(a)`	`a` must start deeper than enclosing block	rules only
`@samecol(a)`	`a` must start at the enclosing column	rules only
`"lit"`	string literal	tokens & rules*
`[a-z]`	character class	tokens only
`[^a-z]`	negated character class	tokens only
`.`	any single character	tokens only
`/pattern/`	regex-literal shorthand	tokens only

* A bare string literal in a rule body is auto-promoted to a compiler-generated anonymous token.

String literal escapes

\n \r \t \\ \" \[ \] \/ \xHH \u{H+} — any other punctuation character escapes to itself (\^, \-, \*, ...).

Case sensitivity

Syntax	Meaning
`"lit"`	case-sensitive, unless `@case_insensitive` is set
`"lit"i`	always case-insensitive
`"lit"cs`	always case-sensitive, even under `@case_insensitive`

Predefined tokens

Token	Default
`DIGIT`	`[0-9]`
`ALPHA`	`[a-zA-Z]`
`ALNUM`	`DIGIT \| ALPHA`
`SPACE`	`[ \t\r\n]`
`HEX`	`[a-fA-F0-9]`

Redeclare any of them (NAME := ...) before first use to override the default. @skip NAME (explicit or default) counts as an immediate use of NAME at that point in the file.

POSIX bracket classes

[:alpha:] [:alnum:] [:digit:] [:space:] [:hex:] — usable inside [...], mixed freely with ranges/literal characters: [[:alpha:]_][[:alnum:]_]*.

Regex-literal shorthand (tokens only)

Supported	Rejected
literals, `.`, `[...]`/`[^...]`, `\|`, `(...)`	anchors `^` `$`
`*` `+` `?` `{n}` `{n,}` `{n,m}`	backreferences `\1`-`\9`
`(?=...)` `(?!...)`	named/unnamed groups `(?<...)`
`\d` `\w` `\s` `\h` `\D` `\W` `\S` `\H`

Maximal munch, in one sentence

At each lexer position: longest match wins; ties go to whichever token was declared first in the file; a zero-width match never wins.

Common gotchas

A literal that's also a substring another token's character class would greedily match (e.g. a keyword "end" vs. an identifier token [a-z]+) ties in length — declare the keyword as its own real token, before the identifier token, so it wins.
@skip NAME (even naming the default SPACE) marks NAME used immediately — you can't redeclare it afterward. Leave the pragma out entirely if you want to redefine the default SPACE's own character set.
Rules can't contain an inline [...], /pattern/, or bare . — give it a token name first.
@samecol checks against the nearest enclosing @indent's column; with no enclosing @indent, that's column 0.

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