In OpenPGP format, "Regular Expression" support in trust signature packages is defined (RFC4880, 18.104.22.168, page 31, and section 8, page 61).
Our implementation of GnuPG just uses regcomp and regexec if system library offers the regexp support. But, here we have interoperability issue; It is not available on Windows. There are corner cases for system's regcomp/regexec. Interoperability issue for trusted signature is considered bad.
And sanitize_regexp routine should be revised/removed, too.