Codeigniter preg_replace（） - 编译失败：消息

I am Posting 5 pages of docx data from cleditor codeigniter ajax. While data passes to the controller it gives warning message:

A PHP Error was encountered

Severity: Warning

Message: preg_replace(): Compilation failed: regular expression is too large 
   at offset 65439

Filename: core/Security.php

Line Number: 641"

and no data posted on controller side like:

// displays nothing
echo $messageText=trim($this->input->post('composeMessage', TRUE));

I tried to set FALSE global_xss_filtering in config.php

$config['global_xss_filtering'] = FALSE;

but it did not work.