The regex that parses header lines potentially causes an unlimited
amount of backtracking, which can cause an exception in the libc++ regex
engine.
The exception that occurs looks like this and is identical to the
message of the exception fixed in
https://github.com/yhirose/cpp-httplib/pull/280:
libc++abi.dylib: terminating with uncaught exception of type
std::__1::regex_error: The complexity of an attempted match
against a regular expression exceeded a pre-set level.
This commit eliminates the problematic backtracking.
libc++ (the implementation of the C++ standard library usually used by
Clang) throws an exception for the regex used by parse_headers before
this patch for certain strings. Work around this by simplifying the
regex and parsing the header lines "by hand" partially. I have repro'd
this problem with Xcode 11.1 which I believe uses libc++ version 8.
This may be a bug in libc++ as I can't see why the regex would result in
asymptotic run-time complexity for any strings. However, it may take a
while for libc++ to be fixed and for everyone to migrate to it, so it
makes sense to work around it in this codebase for now.
HTTP Whitespace and regex whitespace are not the same, so we can't use
\s in regexes when parsing HTTP headers. Instead, explicitly specify
what is considered whitespace in the regex.
