<head> <script type="text/javascript"> function checkthis() { var t=document.forms[0].elements[0]; var reg=/^\d+[.]?\d{1,3}$/; alert(reg.test(t.value)); } </script> </head> <body> <form> <input type="text" id="txt" /> <input type="button" value="檢查" onclick="checkthis();" /> </form> </body>