Trust me, the regex works. I changed my flags to boost::regex_constants::normal, and it all worked just fine, however it does not explain why when I was using just the flags boost::regex_constants::char_classes | boost::regex_constants::intervals, it refused to match more than two words.
The problem is that you need to specify what kind of regular expression it is (basic, extended, Perl etc), if you don't specify anything it defaults to something like POSIX-Basic semantics, which leads to leftmost longest matching being selected. Now onto your expression, the problem here is the [[:space:]]* part: because this can match zero times, your expression could be reduced down to something equivalent to: "([[:alnum:]]+)+" *in the worst case*, and this is the classic "may take forever to match example". It works when you use Perl matching semantics, because the matcher stops as soon as a match is found, if no match is found, then it may well thrash indefinitely (leading to an exception eventually). When POSIX matching semantics are selected, the leftmost longest rule causes the matcher to thrash looking for the "best" possible match, again leading to the exception. So to conclude: specify that you want Perl-style regexes (unless you really want POSIX leftmost longest rules). And, change your expression to something like: "^([[:alpha:]][-[:alnum:]]*(?:[[:space:]]+|$))+$" Hope this helps, John.