BSD Regular Expression Compiling
--------------------------------

  With Berkeley UNIX, you can only search for a given regular
expression; you can't match one.  To search for it, you must first
compile it.  Before you compile it, you must indicate the regular
expression syntax you want it compiled according to by setting the
variable `re_syntax_options' (declared in `regex.h' to some syntax
(see Regular Expression Syntax.).

  To compile a regular expression use:

     char *
     re_comp (char *REGEX)

REGEX is the address of a null-terminated regular expression.
`re_comp' uses an internal pattern buffer, so you can use only the most
recently compiled pattern buffer.  This means that if you want to use a
given regular expression that you've already compiled--but it isn't the
latest one you've compiled--you'll have to recompile it.  If you call
`re_comp' with the null string (*not* the empty string) as the
argument, it doesn't change the contents of the pattern buffer.

  If `re_comp' successfully compiles the regular expression, it returns
zero.  If it can't compile the regular expression, it returns an error
string.  `re_comp''s error messages are identical to those of
`re_compile_pattern' (see GNU Regular Expression Compiling.).