Why doesn't boost regex '.{2}' match '??'

Question

I'm trying to match some chunks if interesting data within a data stream.

There should be a leading

<

then four alphanumeric characters, two characters of checksum (or

??

if no shecksum was specified) and a trailing

>

.

If the last two characters are alphanumeric, the following code works as expected. If they're

??

though it fails.

// Set up a pre-populated data buffer as an example

std::string haystack = "Fli<data??>bble";



// Set up the regex

static const boost::regex e("<\\w{4}.{2}>");

std::string::const_iterator start, end;

start = haystack.begin();

end = haystack.end();

boost::match_flag_type flags = boost::match_default;



// Try and find something of interest in the buffer

boost::match_results<std::string::const_iterator> what;

bool succeeded = regex_search(start, end, what, e, flags); // <-- returns false

I've not spotted anything in the documentation which suggests this should be the case (all but NULL and newline should be match AIUI).

So what have I missed?

Answer 1

Because ??> is a trigraph, it will be converted to }, Your code is equivalence with:

// Set up a pre-populated data buffer as an example
std::string haystack = "Fli<data}bble";

// Set up the regex
static const boost::regex e("<\\w{4}.{2}>");
std::string::const_iterator start, end;
start = haystack.begin();
end = haystack.end();
boost::match_flag_type flags = boost::match_default;

// Try and find something of interest in the buffer
boost::match_results<std::string::const_iterator> what;
bool succeeded = regex_search(start, end, what, e, flags); // <-- returns false

You can change to this, note: I use std::regex which is likely the same:

std::string haystack = "Fli<data?" "?>bble";

DEMO

Why doesn't boost regex '.{2}' match '??'

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