src/parser/generated/regexp-tree.js
function NamedGroupRefOrChars(text, textLoc) {
  const groupName = text.slice(3, -1);

  if (namedGroups.hasOwnProperty(groupName)) {
    return Node({
      type: 'Backreference',
      kind: 'name',
      number: namedGroups[groupName],
      reference: groupName,
    }, textLoc);
  }

  // Else `\k<foo>` should be parsed as a list of `Char`s.
  // This is really a 0.01% edge case, but we should handle it.

  let startOffset = null;
  let startLine = null;
  let endLine = null;
  let startColumn = null;

  if (textLoc) {
    startOffset = textLoc.startOffset;
    startLine = textLoc.startLine;
    endLine = textLoc.endLine;
    startColumn = textLoc.startColumn;
  }

  const charRe = /^[\w$<>]/;
  let loc;

  const chars = [
    // Init to first \k, taking 2 symbols.
    Char(
      text.slice(1, 2),
      'simple',
      startOffset
        ? {
          startLine,
          endLine,
          startColumn,
          startOffset,
          endOffset: startOffset += 2,
          endColumn: startColumn += 2,
        }
        : null
    ),
  ];

  // For \k
  chars[0].escaped = true;

  // Other symbols.
  text = text.slice(2);

  while (text.length > 0) {
    let matched = null;

    // Unicode, \u003B or \u{003B}
    if ((matched = text.match(uRe)) || (matched = text.match(ucpRe))) {
      if (startOffset) {
        loc = {
          startLine,
          endLine,
          startColumn,
          startOffset,
          endOffset: (startOffset += matched[0].length),
          endColumn: (startColumn += matched[0].length),
        };
      }
      chars.push(Char(matched[0], 'unicode', loc));
      text = text.slice(matched[0].length);
    }

    // Simple char.
    else if ((matched = text.match(charRe))) {
      if (startOffset) {
        loc = {
          startLine,
          endLine,
          startColumn,
          startOffset,
          endOffset: ++startOffset,
          endColumn: ++startColumn,
        };
      }
      chars.push(Char(matched[0], 'simple', loc));
      text = text.slice(1);
    }
  }

  return chars;
}
function NamedGroupRefOrChars(text, textLoc) {
  const groupName = text.slice(3, -1);

  if (namedGroups.hasOwnProperty(groupName)) {
    return Node({
      type: 'Backreference',
      kind: 'name',
      number: namedGroups[groupName],
      reference: groupName,
    }, textLoc);
  }

  // Else `\k<foo>` should be parsed as a list of `Char`s.
  // This is really a 0.01% edge case, but we should handle it.

  let startOffset = null;
  let startLine = null;
  let endLine = null;
  let startColumn = null;

  if (textLoc) {
    startOffset = textLoc.startOffset;
    startLine = textLoc.startLine;
    endLine = textLoc.endLine;
    startColumn = textLoc.startColumn;
  }

  const charRe = /^[\w$<>]/;
  let loc;

  const chars = [
    // Init to first \k, taking 2 symbols.
    Char(
      text.slice(1, 2),
      'simple',
      startOffset
        ? {
          startLine,
          endLine,
          startColumn,
          startOffset,
          endOffset: startOffset += 2,
          endColumn: startColumn += 2,
        }
        : null
    ),
  ];

  // For \k
  chars[0].escaped = true;

  // Other symbols.
  text = text.slice(2);

  while (text.length > 0) {
    let matched = null;

    // Unicode, \u003B or \u{003B}
    if ((matched = text.match(uRe)) || (matched = text.match(ucpRe))) {
      if (startOffset) {
        loc = {
          startLine,
          endLine,
          startColumn,
          startOffset,
          endOffset: (startOffset += matched[0].length),
          endColumn: (startColumn += matched[0].length),
        };
      }
      chars.push(Char(matched[0], 'unicode', loc));
      text = text.slice(matched[0].length);
    }

    // Simple char.
    else if ((matched = text.match(charRe))) {
      if (startOffset) {
        loc = {
          startLine,
          endLine,
          startColumn,
          startOffset,
          endOffset: ++startOffset,
          endColumn: ++startColumn,
        };
      }
      chars.push(Char(matched[0], 'simple', loc));
      text = text.slice(1);
    }
  }

  return chars;
}
Extracts from `\k<foo>` pattern either a backreference
to a named capturing group (if it presents), or parses it
as a list of char: `\k`, `<`, `f`, etc.