protected static function getTimestampRegex()
{
return <<<EOF
~^
(?P<year>[0-9][0-9][0-9][0-9])
-(?P<month>[0-9][0-9]?)
-(?P<day>[0-9][0-9]?)
(?:(?:[Tt]|[ \t]+)
(?P<hour>[0-9][0-9]?)
:(?P<minute>[0-9][0-9])
:(?P<second>[0-9][0-9])
(?:\\.(?P<fraction>[0-9]*))?
(?:[ \t]*(?P<tz>Z|(?P<tz_sign>[-+])(?P<tz_hour>[0-9][0-9]?)
(?::(?P<tz_minute>[0-9][0-9]))?))?)?
\$~x
EOF;
}