Not sure if Zend_Filter_Word_SeparatorToCamelCase is what I should be using, but my intention was to filter the string "zend_framework" to "zendFramework".

$obj = new Zend_Filter();
$obj->addFilter(new Zend_Filter_Word_UnderscoreToCamelCase('_'));

echo $obj->filter("zend_framework");  // Prints "Zend_framework"
echo $obj->filter("zend_Framework");  // Prints "ZendFramework"
echo $obj->filter("zendFramework");  // Prints "ZendFramework"
echo $obj->filter("zendframework");  // Prints "Zendframework"
echo $obj->filter("_zendframework");  // Prints "_zendframework"
echo $obj->filter("_zend_framework");  // Prints "_zend_framework"

Zend_Filter_Word_SeparatorToCamelCase

if (self::isUnicodeSupportEnabled()) {
    parent::setMatchPattern(array('#('.$pregQuotedSeparator.')(\p{L}{1})#e','#(^\p{Ll}{1})#e'));
    parent::setReplacement(array("strtoupper('\\2')","strtoupper('\\1')"));
} else {
    parent::setMatchPattern(array('#('.$pregQuotedSeparator.')([A-Z]{1})#e','#(^[a-z]{1})#e'));
    parent::setReplacement(array("strtoupper('\\2')","strtoupper('\\1')"));
}

if (self::isUnicodeSupportEnabled()) {
       parent::setMatchPattern(array('#('.$pregQuotedSeparator.')(\p{L}{1})#e','#(^\p{Ll}{1})#e'));
       parent::setReplacement(array("strtoupper('\\2')","strtolower('\\1')"));
} else {
       parent::setMatchPattern(array('#('.$pregQuotedSeparator.')([a-z]{1})#e','#(^[a-z]{1})#e'));
       parent::setReplacement(array("strtoupper('\\2')","strtolower('\\1')"));
}

{color:green}[A-Z]{color} changed to {color:green}[a-z]{color} {color:green}strtoupper{color} changed to {color:green}strtolower{color}

Not sure if the match pattern for self::isUnicodeSupportEnabled (true) needs to be changed as well.