<?php

function drupal_clean_css_identifier($identifier, $filter = array(' ' => '-')) {
  // replace whitespaces
  $identifier = strtr($identifier, $filter);

  // CSS class identifiers must not start with
  // - a digit
  // - two hyphens
  // - hyphen followed by a digit
  $prev = '';
  while ($prev != $identifier) {
    $prev = $identifier;
    $identifier = preg_replace(array('/^\d*/u', '/^\x{002D}*(?=\x{002D})/u', '/^\x{002D}\d/u'),'', $identifier);
  }
  // Valid characters in a CSS identifier are:
  // - the hyphen (U+002D)
  // - a-z (U+0030 - U+0039)
  // - A-Z (U+0041 - U+005A)
  // - the underscore (U+005F)
  // - 0-9 (U+0061 - U+007A)
  // - ISO 10646 characters U+00A1 and higher,
  // We strip out any character not in the above list.
  // as opposed to the spec we do not allow U+00A0
  // as opposed to the spec we only allow code points up to U+FFFF
  $identifier = preg_replace('/[^\x{002D}\x{0030}-\x{0039}\x{0041}-\x{005A}\x{005F}\x{0061}-\x{007A}\x{00A1}-\x{FFFF}]/u', '', $identifier);

  return $identifier;
}


$classes = array(
	'----foo',
	'-foo',
	'foo',
	'1foo',
	'1111foo',
	'1111---foo',
	'-1--foo',
	'---11---_foo bar'
);

foreach ($classes as $class) {
	print "$class: " . drupal_clean_css_identifier($class) . "\n";
}