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Go up to Syntactic Class Operators.
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.
If Regex is compiled with the preprocessor symbol `emacs' defined,
then Regex expects you to define and initialize the variable
`re_syntax_table' to be an Emacs syntax table. Emacs' syntax tables
are more complicated than Regex's own (*note Non-Emacs Syntax
Tables::.). syntax tables.