Writeup of Rust's lexer

Machine-readable frontmatter grammar

The machine-readable Pest grammar for frontmatter is presented here for convenience.

See Parsing Expression Grammars for an explanation of the notation.

This version of the grammar uses Pest's PUSH, PEEK, and POP for the the matched fences.

ANY, EOI, PATTERN_WHITE_SPACE, XID_START, and XID_CONTINUE are built in to Pest and so not defined below.

FRONTMATTER = {
    WHITESPACE_ONLY_LINE * ~
    START_LINE ~
    CONTENT_LINE * ~
    END_LINE
}

WHITESPACE_ONLY_LINE = {
    ( !"\n" ~ PATTERN_WHITE_SPACE ) * ~
    "\n"
}

START_LINE = {
    PUSH(FENCE) ~
    HORIZONTAL_WHITESPACE * ~
    ( INFOSTRING ~ HORIZONTAL_WHITESPACE * ) ? ~
    "\n"
}

CONTENT_LINE = {
    !PEEK ~
    ( !"\n" ~ ANY ) * ~
    "\n"
}

END_LINE = {
    POP ~
    HORIZONTAL_WHITESPACE * ~
    ( "\n" | EOI )
}

FENCE = { "---" ~ "-" * }

INFOSTRING = {
    ( XID_START | "_" ) ~
    ( XID_CONTINUE | "-" | "." ) *
}

HORIZONTAL_WHITESPACE = { " " | "\t" }


RESERVED = {
    PATTERN_WHITE_SPACE * ~
    FENCE
}