Advanced Syntax Highlighting

So far we've defined some simple syntax highlighting for Potion files: keywords and functions.

If you didn't do the exercises in the last chapter, you need to go back and do them. I'm going to assume you did them.

In fact, you should go back and do any exercises you skipped. Even if you think you don't need them, you need to do them for this book to be effective. Please trust me on this.

Highlighting Comments

One obvious part of Potion that we need to highlight is comments. The problem is that Potion comments start with # which is (almost always) not in iskeyword.

If you don't know what iskeyword means, you didn't listen to me. Go back and do the damn exercises. I'm not just throwing useless busywork at you when I write the exercises for each chapter. You really need to do them to understand the book.

Because # isn't a keyword character we need to use a regular expression to match it (and the rest of the comment). We'll do this with syntax match instead of syntax keyword. Add the following lines to your syntax file:

syntax match potionComment "\v#.*$"
highlight link potionComment Comment

I'm not going to tell you where to put them in the file any more. You're a programmer: use your judgement.

Close and reopen factorial.pn. Add a comment somewhere in the file and you'll see that it's highlighted as a comment.

The second line is simple: it tells Vim to highlight anything in the potionComment syntax group as a Comment.

The first line is something new. We use syntax match which tells Vim to match regexes instead of literal keywords.

Notice that the regular expression we're using starts with \v which tells Vim to use "very magic" mode. Reread the chapter on basic regular expressions if you're not sure what that means.

In this particular case the "very magic" mode isn't necessary. But in the future we might change this regex and wonder why it's not working, so I'd recommend always using "very magic" regexes for consistency.

As for the regex itself, it's fairly simple: comments start with a hash and include all characters from there to the end of the line.

If you need a refresher course on regular expressions you should take a look at Learn Regex the Hard Way by Zed Shaw.

Highlighting Operators

Another part of Potion we need regexes to highlight is operators. Add the following to your syntax file:

syntax match potionOperator "\v\*"
syntax match potionOperator "\v/"
syntax match potionOperator "\v\+"
syntax match potionOperator "\v-"
syntax match potionOperator "\v\?"
syntax match potionOperator "\v\*\="
syntax match potionOperator "\v/\="
syntax match potionOperator "\v\+\="
syntax match potionOperator "\v-\="

highlight link potionOperator Operator

Close and reopen factorial.pn. Notice that the *= in the factorial function is now highlighted.

The first thing you probably noticed about this hunk of code is that I put each regex on its own line instead of grouping them like I did with keywords. This is because syntax match does not support multiple groups on a single line.

You should also note that I used \v at the beginning of every regular expression, even when it wasn't strictly necessary. I prefer to keep my regex syntax consistent when writing Vimscript, even if it means a few extra characters.

You might be wondering why I didn't use a regex like "\v-\=?" to match both - and -= in one line. You can absolutely do that if you want to. It will work just fine. I just tend to think of - and -= as separate operators, so I put them on separate lines.

Defining those operators as separate matches simplifies the regexes at the cost of some extra verbosity. I prefer doing it like that, but you may feel differently. Use your judgement.

I also never defined = as an operator. We'll do that in a second, but I wanted to avoid it for now so I could teach you a lesson.

Because I used separate regexes for - and -= I had to define -= after -!

If I did it in the opposite order and used -= in a Potion file, Vim would match - (and highlight it, of course) and only = would remain for matching. This shows when you're building groups with syntax match each group "consumes" pieces of the file that can't be matched later.

This is an oversimplification, but I don't want to get bogged down in the details just yet. For now, your rule of thumb should be to match larger groups after smaller groups later because groups defined later have priority over groups defined earlier.

Let's go ahead and add = as an operator, now that we've had our lesson:

syntax match potionOperator "\v\="

Take a second and think about where you need to put this in the syntax file. Reread the last few paragraphs if you need a hint.

Exercises

Read :help syn-match.

Read :help syn-priority.

We didn't make : an operator in our example. Read the Potion docs and make a conscious decision about whether to make : an operator. If you decide to do so, add it to the syntax file.

Do the same for . and /.

Add a syntax group potionNumber that highlights numbers. Link it to the highlight group Number. Remember that Potion supports numbers like 2, 0xffaf, 123.23, 1e-2, and 1.9956e+2. Remember to balance the time it takes to handle edge cases with the amount of time those edge cases will actually be used.