It might be desirable to match addresses to regular expressions in rule sets. One way to do this is with the regex class of database map (sequence). If such support is desirable, you can enable inclusion by declaring MAP_REGEX in your Build m4 file like this:

APPENDDEF(`confMAPDEF', `-DMAP_REGEX')

But just defining MAP_REGEX does not guarantee that sendmail will compile with support for it. If you get one of the following errors, or something similar, your C-language library lacks support for the required POSIX regular expression library routines:

undefined reference to 'regcomp'
or
pattern-compile-error: : Operation not applicable
or
ld: Undefined symbol
   _regexec
   _regcomp
   _regerror

If you lack the needed library support, see Section 3.1.3 for instructions on how to download and install regex libraries.

If you are running a precompiled sendmail binary, you can use the -d0.10 debugging command-line switch (-d0.10) to determine if MAP_REGEX support is defined (if it appears in the list, it is defined).