grammar.js

const END_CHARS = [
  ".",
  ",",
  ":",
  ";",
  "!",
  "?",
  "\\",
  "'",
  '"',
  "}",
  "]",
  ")",
  ">",
];

const STOP_CHARS = [
  "/",
  "'",
  '"',
  "<",
  "(",
  "[",
  "{",
  ".",
  ",",
  ":",
  ";",
  "!",
  "?",
  "\\",
  "}",
  "]",
  ")",
  ">",
  // This must be last, so that it isn't interpreted as a range.
  "-",
];

const WHITE_SPACE = /[ \t\f\v]/;

module.exports = grammar({
  name: "comment",

  externals: ($) => [
    $.name,
    $.invalid_token
  ],

  rules: {
    source: ($) => repeat(
      choice(
        $.tag,
        $.modeline,
        $._full_uri,
        alias($._text, "text"),
      ),
    ),

    tag: ($) => seq(
      $.name,
      optional($._user),
      ":",
    ),

    modeline: ($) => prec.right(choice(
      // First form
      seq(
        choice("vi:", "vim:", "ex:"),
        $.option,
        repeat(seq(choice(repeat1(WHITE_SPACE), ':'), $.option)),
        optional(choice(repeat1(WHITE_SPACE), ':')),
      ),
      // Second form
      seq(
        choice("vi:", "vim:", "ex:"),
        /set? /,
        repeat1($.option),
        ":",
      ),
      seq(
        "Vim:",
        /set /,
        repeat1($.option),
        ":",
      ),
    )),

    option: ($) => /[^\s:]+/,

    _user: ($) => seq(
      "(",
      alias(/[^()]+/, $.user),
      ")",
    ),

    // This token is split into two parts so the end character isn't included in the URI itself.
    _full_uri: ($) => seq($.uri, choice(alias($._end_char, "text"), /\s/)),

    // This token needs to be single regex, otherwise a partial match will result in an error.
    uri: ($) => get_uri_regex(),

    // Text tokens can be a single character, or a sequence of characters that aren't stop characters.
    _text: ($) => choice($._stop_char, notmatching(STOP_CHARS)),
    _stop_char: ($) => choice(...STOP_CHARS),
    _end_char: ($) => choice(...END_CHARS),
  },
});

/**
 * Get a regex that matches a URI.
 *
 * A URI matches if:
 *
 * - It starts with http:// or https://
 * - It contains at least one character that isn't whitespace or an end character.
 * - If it contains an end character, it must be followed by a letter or number (.com).
 * - It doesn't end with a whitespace or an end character (this marks the end of the URI).
 *
 * An end character is a character that marks the end of a sentence.
 */
function get_uri_regex() {
  let end_chars = escapeRegExp(END_CHARS.join(""));
  return new RegExp(
    `https?://([^\\s${end_chars}]|[${end_chars}][a-zA-Z0-9])+`
  );
}

/**
 * Match any characters that aren't whitespace or that aren't in the given list.
 */
function notmatching(chars) {
  chars = escapeRegExp(chars.join(""));
  return new RegExp(`[^\\s${chars}]+`);
}

/**
 * Escape a string for use in a regular expression.
 *
 * Taken from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions#escaping.
 */
function escapeRegExp(string) {
  return string.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
}