The Knot-Cords of the Drennick Outriders

The Drennick Outriders ride the long patrols beyond Bitwick and send word home on knotted cords. At the Outrider post, scribes copy each cord into a shorthand, but they copy carelessly. They split one run of knots across several groups, they write needless counts of 1, and sometimes they botch the notation entirely. The Cord-Warden of the post pays a standing bounty for every cord rewritten in the one canonical shorthand, and for every botched cord caught.

The shorthand. A cord is a string of printable ASCII characters. It is read left to right as a sequence of *groups*. Each group is an optional *count* followed by exactly one *symbol*:

• A count is all the consecutive digits 0-9 at the start of a group, read as a decimal number. It must not begin with 0. A group with no digits has count 1.
• A symbol is one of two things. It may be a single character that is neither a digit nor a backslash; spaces and punctuation count as symbols. Or it may be a backslash followed by a digit or by a second backslash, and it then stands for that digit or for a backslash.
• A group stands for its symbol's character repeated *count* times. The message is the characters of all groups, concatenated in order.

A cord is malformed if any of the following is true:

• a count begins with 0;
• digits run to the end of the cord with no symbol after them;
• a backslash is the last character;
• a backslash is followed by anything other than a digit or a backslash.

The canonical form. Split the message into maximal runs of identical characters. Write each run, in order, as one group:

• First its length, as a decimal number with no leading zeros. Omit the length entirely when it is 1.
• Then its character. Escape the character with a backslash if it is a digit or a backslash. Otherwise write it plainly.

Write retieCord(cord). Given cord (a string), it returns the canonical form of that cord as a string. If the cord is malformed, it returns null in TypeScript and None in Python. The empty cord is well-formed, and its canonical form is the empty string.

Each count is at most 1,000,000,000 and a cord has at most 1,000 characters. A message may therefore be far too long to write out in full.