Much faster version of utf8_isASCII()

Ignore-this: 1adbc2b33e76b3a76e650c340e9644e6
This version uses a non-capturing regular expression instead of looping through
all characters of the string.

darcs-hash:20090724084009-2cb76-ad1630c7aca53f0bdb596525b0693304a9b4cc88.gz