Hello. I have a file input in a form that uploads a mp3 file, but I´d like to detect conflictive characters to my system in the filename, like ! @ or any other. All codes I´ve found replace these characters, but I just want to detect them to alert the user. I think it will be easy with regular expressions, but I dont know about them. I´m using jquery/javascript. Thanks in advance for your help Edit to improve my problem description: I´m working in a CodeIgniter application that allows user to upload mp3 files to the server. I use jQuery to manage client side forms. The CI upload class converts spaces in the file name to underscores and everything works. But testing the application I uploaded a mp3 file with a (!) in the name, and I got troubles with it. I just want to insert a javascript conditional before the file is uploaded to evaluate if the user´s filename contains a (!) (or any other I´d like to add later) to ask for the file to be renamed if it does.