18.6.2.1 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).