http://lex.clansfx.co.uk/projects/regexbugtest.d
My workaround was to list all the characters \s is meant to match. Note that I have a bigger example of this bug if you're interested.
Minimal example to avoid wasting your time:
module main;
import std.stdio,std.regex;
void main()
{
//minimal regex; should match full "sec" but matches "s" instead
//regextester.com handles it properly
foreach(m; match("sec", regex(`([\s_]|sec)`, "gi")))
writefln("%s[%s]%s", m.pre, m.hit, m.post);
return;
}