怎么把PascalCase转换成pascal_case？

尝试以下尺寸：

$tests = array(  'simpleTest' => 'simple_test',  'easy' => 'easy',  'HTML' => 'html',  'simpleXML' => 'simple_xml',  'PDFLoad' => 'pdf_load',  'startMIDDLELast' => 'start_middle_last',  'AString' => 'a_string',  'Some4Numbers234' => 'some4_numbers234',  'TEST123String' => 'test123_string',);foreach ($tests as $test => $result) {  $output = from_camel_case($test);  if ($output === $result) {    echo "Pass: $test => $resultn";  } else {    echo "Fail: $test => $result [$output]n";  }}function from_camel_case($input) {  preg_match_all('!([A-Z][A-Z0-9]*(?=$|[A-Z][a-z0-9])|[A-Za-z][a-z0-9]+)!', $input, $matches);  $ret = $matches[0];  foreach ($ret as &$match) {    $match = $match == strtoupper($match) ? strtolower($match) : lcfirst($match);  }  return implode('_', $ret);}

输出：

Pass: simpleTest => simple_testPass: easy => easyPass: HTML => htmlPass: simpleXML => simple_xmlPass: PDFLoad => pdf_loadPass: startMIDDLELast => start_middle_lastPass: AString => a_stringPass: Some4Numbers234 => some4_numbers234Pass: TEST123String => test123_string

这实现了以下规则：

1. 以小写字母开头的序列后必须跟小写字母和数字；
2. 以大写字母开头的序列后面可以是：
   • 一个或多个大写字母和数字（后跟字符串的结尾或大写字母，后跟小写字母或数字，即下一个序列的开头）；要么
   • 一个或多个小写字母或数字。