3 Pitfalls and sand in the gears
As compact and expressive as regular expressions can be, there are times when they simply go disastrously wrong. Be careful to avoid pitfalls, and at least understand and identify where such difficulties arise.
Summary
Use regular expressions to validate a message protocol quickly.
In this puzzle, we imagine a certain message protocol (as we do in many of the other puzzles). We have a message alphabet that consists of the following symbols:
| Codepoint |
Name |
Appearance |
|---|---|---|
| U+25A0 |
Black Square |
■ |
| U+25AA |
Black Small Square |
▪ |
| U+25CB |
White Circle |
○ |
| U+25C9 |
Fisheye |
◉ |
| U+25A1 |
White Square |
□ |
| U+25AB |
White Small Square |
▫ |
| U+25B2 |
Black Up Triangle |
▲ |
| U+25CF |
Black Circle |
● |
| U+2404 |
End Transmition |
␄ (! herein) |
These geometric characters are attractive and are chosen to avoid thinking of matches in terms of natural language words that some other puzzles utilize. However, feel free in solving it to substitute letters or numerals, which are probably easier to type in your shell. As long as the correspondences are one-to-one, it does not matter what symbols are used.
Imagine that these symbols are part of a message protocol. In this protocol, valid messages consist of alternating blocks belonging to either “type 1” or “type 2”. Each message must also end with an “end of transmission character.”