Emacs Lisp: Syntax Table

What is Syntax Table

Emacs has a concept of Syntax Table. The basic idea is, each character (every Unicode character), is categorized into a class. Classes are: letters, punctuations, brackets, programing language identifiers, comment character, string delimiters, etc.

Where is Syntax Table Used

Syntax table is heavily used in emacs. For example,

• Most cursor movement commands rely on syntax table. For example, when you Alt+x forward-word 【Alt+f】, emacs will move cursor until it reaches a character that's not in the “word” class.
• Syntax coloring of strings and comments rely on syntax table. [see Emacs Lisp: Syntax Color Comment in Major Mode]
• Programing language comment/uncomment command also relies on syntax table. [see Emacs Lisp: How to Write Comment/Uncomment Command for a Major Mode]
• Lisp mode's parenthesis navigation also depends on syntax table. [see Emacs: Navigate Lisp Code as Tree]

Syntax Table is Local to Buffer

Each buffer has its own version of syntax table. Typically, when a Major Mode is activated, it changes the current buffer's syntax table.

View Current Syntax Table

Alt+x describe-syntax to show current buffer's syntax table.

Syntax Classes

Each syntax class is identified by a 1-char code.

Here's the most important character classes and their 1-char code.

For complete list, see Syntax Class Table (ELISP Manual)

Syntax Descriptor

A syntax descriptor is a lisp string that specifies a syntax class (the 1-char code), a matching character (used only for characters in the bracket class) and flags (used for comment delimiters).

Syntax Descriptors (ELISP Manual)

You'll use Syntax Descriptor with the function modify-syntax-entry, when you create a syntax table. Here's a quick example.

;; make period to have syntax class of symbol
(modify-syntax-entry ?. "_" my-syn-table)

;; make the «French double quote» to be brackets
(modify-syntax-entry ?« "(»" my-syn-table)
(modify-syntax-entry ?» ")«" my-syn-table)

;; make char's syntax for C++ style comment “// …”
(modify-syntax-entry ?/ ". 12b" my-syn-table)
(modify-syntax-entry ?\n "> b" my-syn-table)

Now let's look at the line

(modify-syntax-entry ?. "_" my-syn-table)

modify-syntax-entry take 3 args.

1. A character. [see Emacs Lisp: Character Type]
2. A syntax descriptor string.
3. A variable whose value is a syntax table.

?. is the character period . (U+2E: FULL STOP).

"_" is the syntax descriptor string. It means the period character is in symbol class.

Now let's look at this line:

(modify-syntax-entry ?« "(»" my-syn-table)

It means, the character « is in the class of opening bracket, and its matching character is ».

Now, this line:

(modify-syntax-entry ?/ ". 12b" my-syn-table)

Let's look at the syntax descriptor string ". 12b". It means:

• The first character in syntax descriptor is period. It means the slash / character is punctuation class.
• The second character in syntax descriptor is space. Since it's not in bracket class, so there is no matching character, space character is used to mean none.
• The rest characters in syntax descriptor is flags. They are used for comment class character. Because the comment delimiter characters in programing languages is complex. The char usually have multiple purposes, and also depends on whether the character is repeated, or if it can be nested.

The flags in syntax descriptor is very complex. See elisp manual Syntax Flags (ELISP Manual)

For practical examples of using the syntax flags for comments, see Emacs Lisp: Syntax Color Comment in Major Mode

Basic Facts of Syntax Table

• Syntax table is a lookup table, and is implemented as a special vector. You use make-syntax-table and others to create it.
• Each buffer has its own syntax table. (so, it's like a Buffer Local Variable, but there is no variable.)
• Use set-syntax-table to set a syntax table for current buffer.

Create Syntax Table

Here's typical way to create a syntax table.

;; typical way to create and set syntax table

(defvar xpy-mode-syntax-table nil "Syntax table for `xpy-mode'.")

(setq xpy-mode-syntax-table
      (let ( (synTable (make-syntax-table)))

        ;; set/modify each char's class
        (modify-syntax-entry ?# "<" synTable)
        (modify-syntax-entry ?\n ">" synTable)
        ;; more lines here ...

        ;; return it
        synTable))

;; then, have this line inside your mode definition. So that, when user calls your major mode, it will set syntax table for whatever is the current buffer of user
(set-syntax-table xpy-mode-syntax-table)

Standard Syntax Table

• The function standard-syntax-table returns the standard syntax table.
• Standard syntax table is the syntax table used by fundamental-mode. [see Emacs: What is Major Mode]
• Every syntax table is derived from standard syntax table.

Syntax Table Inheritance

Emacs syntax table has inheritance. That is, each syntax table you create inherits a parent syntax table. You do not need to set every character's syntax class. When a syntax table does not have entry for a character, it uses the parent table.

Syntax table commands have optional parameter for a table name of a parent table. When not specified, the parent syntax table is the standard syntax table.

Syntax Table Functions (ELISP Manual)