x86: fix build on non-C locales.

For some locales regex range [a-zA-Z] does not work as it is supposed to.
so we have to use [:alnum:] and [:xdigit:] to make it work as intended.

[1] http://en.wikipedia.org/wiki/Estonian_alphabet

Signed-off-by: Ingo Molnar <mingo@elte.hu>