Function pathToPattern

packages/fresh/src/router.ts:187–298

(
  path: string,
  options?: { keepGroups: boolean },
)

 * Transform a filesystem URL path to a `path-to-regex` style matcher.
 */
export function pathToPattern(
  path: string,
  options?: { keepGroups: boolean },
): string {
  const parts = path.split("/");
  if (parts[parts.length - 1] === "index") {
    if (parts.length === 1) {
      return "/";
    }
    parts.pop();
  }

  let route = "";

  let nonOptionalSegments = 0;
  for (let i = 0; i < parts.length; i++) {
    const part = parts[i];

    // Case: /[...foo].tsx
    if (part.startsWith("[...") && part.endsWith("]")) {
      route += `/:${part.slice(4, part.length - 1)}*`;
      continue;
    }

    // Route groups like /foo/(bar) should not be included in URL
    // matching. They are transparent and need to be removed here.
    // Case: /foo/(bar) -> /foo
    // Case: /foo/(bar)/bob -> /foo/bob
    // Case: /(foo)/bar -> /bar
    if (!options?.keepGroups && part.startsWith("(") && part.endsWith(")")) {
      continue;
    }

    // Disallow neighbouring params like `/[id][bar].tsx` because
    // it's ambiguous where the `id` param ends and `bar` begins.
    if (part.includes("][")) {
      throw new SyntaxError(
        `Invalid route pattern: "${path}". A parameter cannot be followed by another parameter without any characters in between.`,
      );
    }

    // Case: /[[id]].tsx
    // Case: /[id].tsx
    // Case: /[id]@[bar].tsx
    // Case: /[id]-asdf.tsx
    // Case: /[id]-asdf[bar].tsx
    // Case: /asdf[bar].tsx
    let pattern = "";
    let groupOpen = 0;
    let optional = false;
    for (let j = 0; j < part.length; j++) {
      const char = part[j];
      if (char === "[") {
        if (part[j + 1] === "[") {
          // Disallow optional dynamic params like `foo-[[bar]]`
          if (part[j - 1] !== "/" && !!part[j - 1]) {
            throw new SyntaxError(
              `Invalid route pattern: "${path}". An optional parameter needs to be a full segment.`,