jquery - Allow only space, underscore , alphabets and numbers in text box -

i have code allows characters in text box. allows alphabets , numbers first character. now, how allow space , underscore along alphabets , numbers? want code in jquery. found similar solution on here doesn't contain jquery code. http://jsfiddle.net/fwcfq/39/

    $('#value').bind('keypress', function(e) {         if($('#value').val().length == 0){             if (e.which == 32){//space bar                 e.preventdefault();             }             var valid = (e.which >= 48 && e.which <= 57) || (e.which >= 65 && e.which     <= 90) || (e.which >= 97 && e.which <= 122);             if (!valid) {                e.preventdefault();             }         }     });  

this code want

$('#value').bind('keypress', function (e) {     if ($('#value').val().length == 0) {         if (e.which == 32) { //space bar             e.preventdefault();         }         var valid = (e.which >= 48 && e.which <= 57) || (e.which >= 65 && e.which <= 90) || (e.which >= 97 && e.which <= 122);         if (!valid) {             e.preventdefault();         }     } else {         var valid = (e.which >= 48 && e.which <= 57) || (e.which >= 65 && e.which <= 90) || (e.which >= 97 && e.which <= 122 || e.which == 32 || e.which == 95 || e.which == 8);         if (!valid) {             e.preventdefault();         }     } }); 

fiddle

this approach assumes know keys want accept however. instance enter key rejected unless explicitly allow it.