Regex to validate user input (match any word or phrase)

I am trying to test user input to ensure it contains special character or
symbols that could cause problems to my program or database.

The user will enter a keyword in the web form to search in my database, and
I want to ensure I don't play around with special character otherwise any
word or phrase is valid.

I have changed my regex several times, but still doesn't work and now I have
something bizarre and it doesn't work either. 


$string !~ /^[A-Za-z0-9]+/ # did not work when 2 words are supplied. 
$string !~ /(^[A-Za-z0-9])+\s*$1/ # did not work.

if ($string =~ /!|\$|%|\?|\^|\+|@|'|\\||/){ # doesn't work.

        print qq(<p style="font-weight: bold;">Invalid Keyword</p>);
        print qq(<p>You have entered an invalid search keyword.
        <br /> Please go back and enter a valid word.</p>);
        footer();
        exit;

    }

Any idea how to match this properly?

Mimi

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