function is_heb(Field) {
        // First choose the required validation

        HebrewChars = new RegExp("^[\u0590-\u05FF]+$");
        AlphaNumericChars = new RegExp("^[a-zA-Z0-9\-]+$");
        EnglishChars = new RegExp("^[a-zA-Z\-]+$");
        LegalChars = new RegExp("^[a-zA-Z\-\u0590-\u05FF ]+$"); //Note that this one allows space 

        // Then use it

        if (!LegalChars.test(Field)) {
            return false;
        } else
            return true;
    }

<input id="the_text" type="text" value="בדיקה" />
<br /><button onclick="document.getElementById('the_result').value = is_heb(document.getElementById('the_text').value)">Is it Hebrew?</button>
<br /><br />
Result:
<br /><input id="the_result" type="text">