document regex syntax

[jjergus]

We should have a guide that documents:

• re"..." literals (Hack-specific feature)
• overview of the HH\Lib\Regex namespace from HSL, link to relevant API docs pages
• link to an authoritative reference for PCRE regex syntax (e.g. https://www.pcre.org/original/doc/html/pcresyntax.html)
• delimiter syntax (inherited from PHP: https://www.php.net/manual/en/regexp.reference.delimiters.php)
• modifier syntax (also inherited from PHP: https://www.php.net/manual/en/reference.pcre.pattern.modifiers.php -- but we don't support all of them so need our own list)
  • source: https://github.com/facebook/hhvm/blob/master/hphp/runtime/base/preg.cpp#L775

[fredemmott]

List of supported modifiers at https://github.com/facebook/hhvm/blob/master/hphp/runtime/base/preg.cpp

delimiter/modifier syntax largely inherited from perl - e.g. also in perl, ruby (%r), vim, etc