14.4.1.1 Non-Emacs Syntax Tables

A syntax table is an array indexed by the characters in your character set. In the ASCII encoding, therefore, a syntax table has 256 elements. Regex always uses a char * variable re_syntax_table as its syntax table. In some cases, it initializes this variable and in others it expects you to initialize it.

• If Regex is compiled with the preprocessor symbols emacs and SYNTAX_TABLE both undefined, then Regex allocates re_syntax_table and initializes an element i either to Sword (which it defines) if i is a letter, number, or ‘_’, or to zero if it’s not.
• If Regex is compiled with emacs undefined but SYNTAX_TABLE defined, then Regex expects you to define a char * variable re_syntax_table to be a valid syntax table.
• See Emacs Syntax Tables, for what happens when Regex is compiled with the preprocessor symbol emacs defined.