Refactor regex code

[WGH-]

While proceeding with preparation of my RE2 patch (#1996), I found quite a few odd things with regex code, and decided to attempt to fix them first:

• Lack of encapsulation.
• Non-method functions for matching without retrieving all groups.
• Regex class being copyable without proper copy-constructor (potential UAF
  and double free due to pointer members m_pc and m_pce).
• Redundant SMatch::m_length, which always equals to match.size() anyway.
• Weird SMatch::size_ member which is initialized only by one of the three matching
  functions, and equals to the return value of that function anyways.
• Several places in code having std::string value instead of reference.

[zimmerle]

Hi @WGH-,

Thank you for the patch. Having a look at it tomorrow.

[zimmerle]

Patch merged! Made minimal cosmetic changes to make the style check script happy again.

$ make check-coding-style | grep regex 
warning: ./src/utils/regex.cc:43:  { should almost always be at the end of the previous line  [whitespace/braces] [4]
warning: ./src/utils/regex.cc:112:  Closing ) should be moved to the previous line  [whitespace/parens] [2]

I do have suggestions for more refactoring surround this particular implementation. I will send it over you via patch.

[mdunc]

I think this caused my build to fail when compiling ModSecurity for nginx. Configured with the --with-lmdb option.

collection/backend/lmdb.cc: In member function ‘virtual void modsecurity::collection::backend::LMDB::resolveRegularExpression(const string&, std::vector<const modsecurity::VariableValue*>*, modsecurity::Variables::KeyExclusions&)’:
collection/backend/lmdb.cc:541:38: error: use of deleted function ‘modsecurity::Utils::Regex::Regex(const modsecurity::Utils::Regex&)’

Edit: Created an issue for this #2008