At the moment we use 'fromUTF32' which is only in ICU >=4.1.4
[springmeyer] This looks like it was added to ICU one year ago: http://bugs.icu-project.org/trac/ticket/6677
[springmeyer] it was assigned to milestone 4.1.4, so I'm assuming that is the min release.
[springmeyer] r1597 disables regex support for ICU versions lacking fromUTF32. needs work...
[springmeyer] added as a configure time check in r2431, re-assigning to artem to take a look and close if it is adequate (and if we do not instead wish to add back support for earlier versions of icu)