Parsing ANSI escape codes?

Question

I\'m building a telnet app in C# (for scripting door games on oldschool BBS systems e.g. Wildcat) and can\'t seem to build a working parser for ANSI escape codes (e.g. cursor mo

Answer 1

A proper answer depends on how one intends to use the library. Any terminal emulator will read those sequences and perform actions based on them. But even a simple terminal emulator will understand about a hundred sequences.

Your example, in a perhaps more readable form, looks like this:

\E[s
\E[255B
\E[255C\t_
\E[6n
\E[u
\E[!_^B^B?HTML?
\E[0m_
\E[2J
\E[H\f\r
\n

using unmap (making the escape character \E and showing all characters printable — and beginning a new line for escape characters).

ECMA-48 describes the format for

• single-byte control characters, and
• multibyte control sequences (beginning with the escape character).

Control sequences have content (parameters) which are limited to certain characters such as digits and separators, e.g., ';'. Control sequences also have a definite ending, called the final character. The sequence \E[!_^B^B? does not follow those rules. As suggested in a comment, perhaps your recording was confused by the terminal's response to the cursor position request \E[6n.

With that much context:

• some of the actions performed by a terminal emulator modify the display (\E[2J clears the display)
• some of the actions performed by a terminal emulator tell the host about the display (\E[6n asks the terminal where the cursor is)
• some of the actions performed by a terminal emulator modify the terminal's behavior (\E[s and \E[u save the cursor position and restore it later)

In short, you may see that to process the control sequences received by a terminal, you really need a terminal program to do all of this. Not all terminal emulators are the same, however. Some use a series of case-statements, to handle the successive stages of escape, bracket, digits, etc. But your program should keep in mind that single-byte controls can appear in the middle of multi-byte control sequences. Since they are encoded differently, there is no conflict. But it makes the program more complicated than you might suppose for just reading one sequence at a time.

xterm uses some case-statements (for the final character, basically), but most of the state transitions in decoding a control sequence are done using a set of tables. They are very repetitive, but not obvious to construct: Paul Williams pointed out that for a VT100, those should be symmetric (essentially treating the input as 7-bit ASCII). Some of the states are treated as errors, and ignored; well-formatted sequences are all that matters anyway. In theory, you could reuse the state-tables and add a "little" parsing. The tables are 8500 lines (one state per line).

Aside from (a) reading existing terminal emulators and imitating them on a smaller scale, or (b) modifying a terminal emulator ... you could investigate libvterm:

An abstract C99 library which implements a VT220 or xterm-like terminal emulator. It doesn't use any particular graphics toolkit or output system, instead it invokes callback function pointers that its embedding program should provide it to draw on its behalf. It avoids calling malloc() during normal running state, allowing it to be used in embedded kernel situations.

However, that is not in C# (and the source is the documentation). Still, it is only 5500 lines of code.

Further reading:

• Comparing versions, by counting controls
• XTerm Control Sequences
• A parser for DEC’s ANSI-compatible video terminals
• ECMA-48: Control Functions for Coded Character Sets